Dear Allin and Jack, I think there is a mistake in the mwrite funtion help. According to it: "If fname has the suffix ".gz" the the file is written with gzip compression. The format is basically as described above except that the matrix elements are written in a single column, in column-major order." Running the following script and openning testPLAIN and testGZ in a text editor I get the same output (the matrix elements are not written in a single column): M = {1,2; 3,4; 5,6; 7,8; 9,10} mwrite(M, "testPLAIN") mwrite(M, "testGZ.gz") Best regards, Henrique Andrade