One way to do it would be to make a subset from one of the files first, in whatever way you normally do. For example, using Basic Tools > Resize Data, and choosing the Spatial Subset button. Then when you go to make the subset of the second file, using Basic Tools > Resize Data, you (again) choose the Spatial Subset button, and in the Select Spatial Subset dialog, under the Subset Using section, click the file button. Choose the subset that you just made from the first file here, which tells ENVI to match this subset from the second file to that subset from the first file. Your two subsets should then have the same dimensions and extent. - Peg