FreeMat does not want to read *.mat files, where it gives the following mistake: "The specific error was: Not a valid MAT file". After playing around with it for a little bit I have realized that the specific *.mat files that can not be read are the once that are converted with Stat Transfer from another format (for example from *.dta to *.mat). The *.mat files that are saved directly from Matlab are still OK.