diff --git a/cpp/Matrix.cpp b/cpp/Matrix.cpp
index 114ddd3e2..fa6d6dbe7 100644
--- a/cpp/Matrix.cpp
+++ b/cpp/Matrix.cpp
@@ -494,24 +494,26 @@ Matrix stack(size_t nrMatrices, ...)
 }
 
 /* ************************************************************************* */
-Matrix collect(vector<const Matrix *> matrices)
+Matrix collect(const std::vector<const Matrix *>& matrices, size_t m, size_t n)
 {
-  int dimA1 = 0;
-  int dimA2 = 0;
-  BOOST_FOREACH(const Matrix* M, matrices) {
-    dimA1 =  M->size1();  // TODO: should check if all the same !
-    dimA2 += M->size2();
-  }
-  Matrix A(dimA1, dimA2);
-  int hindex = 0;
-  BOOST_FOREACH(const Matrix* M, matrices) {
-    for(size_t d1 = 0; d1 < M->size1(); d1++)
-      for(size_t d2 = 0; d2 < M->size2(); d2++)
-	A(d1, d2+hindex) = (*M)(d1, d2);
-    hindex += M->size2();
-  }  
+	// if we have known and constant dimensions, use them
+	size_t dimA1 = m;
+	size_t dimA2 = n*matrices.size();
+	if (!m && !n)
+		BOOST_FOREACH(const Matrix* M, matrices) {
+		dimA1 =  M->size1();  // TODO: should check if all the same !
+		dimA2 += M->size2();
+	}
+	Matrix A(dimA1, dimA2);
+	size_t hindex = 0;
+	BOOST_FOREACH(const Matrix* M, matrices) {
+		for(size_t d1 = 0; d1 < M->size1(); d1++)
+			for(size_t d2 = 0; d2 < M->size2(); d2++)
+				A(d1, d2+hindex) = (*M)(d1, d2);
+		hindex += M->size2();
+	}
 
-  return A;
+	return A;
 }
 
 /* ************************************************************************* */
diff --git a/cpp/Matrix.h b/cpp/Matrix.h
index dcded7637..723523013 100644
--- a/cpp/Matrix.h
+++ b/cpp/Matrix.h
@@ -252,9 +252,14 @@ Matrix stack(size_t nrMatrices, ...);
 /**
  * create a matrix by concatenating
  * Given a set of matrices: A1, A2, A3...
+ * If all matrices have the same size, specifying single matrix dimensions
+ * will avoid the lookup of dimensions
+ * @param matrices is a vector of matrices in the order to be collected
+ * @param m is the number of rows of a single matrix
+ * @param n is the number of columns of a single matrix
  * @return combined matrix [A1 A2 A3]
  */
-Matrix collect(std::vector<const Matrix *> matrices);
+Matrix collect(const std::vector<const Matrix *>& matrices, size_t m = 0, size_t n = 0);
 Matrix collect(size_t nrMatrices, ...);
 
 /**
diff --git a/cpp/testMatrix.cpp b/cpp/testMatrix.cpp
index f75bf80c7..0b26f75d4 100644
--- a/cpp/testMatrix.cpp
+++ b/cpp/testMatrix.cpp
@@ -72,23 +72,61 @@ TEST( matrix, row_major )
 }
 
 /* ************************************************************************* */
-TEST( matrix, collect )
+TEST( matrix, collect1 )
 {
-  Matrix A = Matrix_(2,2, 
-		       -5.0 , 3.0,
-		       00.0, -5.0 );
-  Matrix B = Matrix_(2,3,
-		       -0.5 , 2.1, 1.1, 
-		       3.4 , 2.6 , 7.1);
-  Matrix AB = collect(2, &A, &B);
-  Matrix C(2,5);
-  for(int i = 0; i < 2; i++) for(int j = 0; j < 2; j++) C(i,j) = A(i,j);
-  for(int i = 0; i < 2; i++) for(int j = 0; j < 3; j++) C(i,j+2) = B(i,j);
+	Matrix A = Matrix_(2,2,
+			-5.0 , 3.0,
+			00.0, -5.0 );
+	Matrix B = Matrix_(2,3,
+			-0.5 , 2.1, 1.1,
+			3.4 , 2.6 , 7.1);
+	Matrix AB = collect(2, &A, &B);
+	Matrix C(2,5);
+	for(int i = 0; i < 2; i++) for(int j = 0; j < 2; j++) C(i,j) = A(i,j);
+	for(int i = 0; i < 2; i++) for(int j = 0; j < 3; j++) C(i,j+2) = B(i,j);
 
-  EQUALITY(C,AB);
+	EQUALITY(C,AB);
 
 }
 
+/* ************************************************************************* */
+TEST( matrix, collect2 )
+{
+	Matrix A = Matrix_(2,2,
+			-5.0 , 3.0,
+			00.0, -5.0 );
+	Matrix B = Matrix_(2,3,
+			-0.5 , 2.1, 1.1,
+			3.4 , 2.6 , 7.1);
+	vector<const Matrix*> matrices;
+	matrices.push_back(&A);
+	matrices.push_back(&B);
+	Matrix AB = collect(matrices);
+	Matrix C(2,5);
+	for(int i = 0; i < 2; i++) for(int j = 0; j < 2; j++) C(i,j) = A(i,j);
+	for(int i = 0; i < 2; i++) for(int j = 0; j < 3; j++) C(i,j+2) = B(i,j);
+
+	EQUALITY(C,AB);
+
+}
+
+/* ************************************************************************* */
+TEST( matrix, collect3 )
+{
+	Matrix A, B;
+	A = eye(2,3);
+	B = eye(2,3);
+	vector<const Matrix*> matrices;
+	matrices.push_back(&A);
+	matrices.push_back(&B);
+	Matrix AB = collect(matrices, 2, 3);
+	Matrix exp = Matrix_(2, 6,
+			1.0, 0.0, 0.0, 1.0, 0.0, 0.0,
+		    0.0, 1.0, 0.0, 0.0, 1.0, 0.0);
+
+	EQUALITY(exp,AB);
+}
+
 /* ************************************************************************* */
 TEST( matrix, stack )
 {
diff --git a/cpp/timeMatrix.cpp b/cpp/timeMatrix.cpp
index 201d465a8..3ba3281d2 100644
--- a/cpp/timeMatrix.cpp
+++ b/cpp/timeMatrix.cpp
@@ -5,20 +5,24 @@
  */
 
 #include <iostream>
-#include <boost/progress.hpp>
+#include <boost/timer.hpp>
 #include "Matrix.h"
 
 using namespace std;
 using namespace gtsam;
 
-int main(int argc, char ** argv) {
-	cout << "Starting Matrix::collect() Timing" << endl;
-
+/*
+ * Results:
+ * Alex's Machine:
+ *  - no pass: 0.1818 sec
+ *  - pass   : 0.1802 sec
+ */
+double timeCollect(size_t p, size_t m, size_t n, bool passDims, size_t reps) {
 	// create a large number of matrices
-	size_t n, m, p;
-	p = 100000; // number of matrices
-	n = 10;  // rows per matrix
-	m = 12;  // columns per matrix
+	// p =  number of matrices
+	// m =  rows per matrix
+	// n =  columns per matrix
+	// reps = number of repetitions
 
 	// fill the matrices with identities
 	vector<const Matrix *> matrices;
@@ -30,15 +34,37 @@ int main(int argc, char ** argv) {
 
 	// start timing
 	Matrix result;
+	double elapsed;
 	{
-		boost::progress_timer t;
-		result = collect(matrices);
-	}
+		boost::timer t;
 
+		if (passDims)
+			for (int i=0; i<reps; ++i)
+				result = collect(matrices, m, n);
+		else
+			for (int i=0; i<reps; ++i)
+				result = collect(matrices);
+		elapsed = t.elapsed();
+	}
 	// delete the matrices
 	for (int i=0; i<p;++i) {
 		delete matrices[i];
 	}
 
+	return elapsed/reps;
+}
+
+int main(int argc, char ** argv) {
+
+	// Time collect()
+	cout << "Starting Matrix::collect() Timing" << endl;
+	size_t p = 100000; size_t m = 10; size_t n = 12; size_t reps = 50;
+	double collect_time1 = timeCollect(p, m, n, false, reps);
+	double collect_time2 = timeCollect(p, m, n, true, reps);
+	cout << "Elapsed time for collect (no pass) [" << p << " (" << m << ", " << n << ") matrices] : " << collect_time1 << endl;
+	cout << "Elapsed time for collect (pass)    [" << p << " (" << m << ", " << n << ") matrices] : " << collect_time2 << endl;
+
+
+
 	return 0;
 }