PtxCo(1-x) thin films where 0.15 less than or equal to x less than or equal to 0.35 have a potential use in ultra high density longitudinal recording and have been studied using various TEM techniques. The thin films have been deposited onto glass, Si/Si3N4 and Si3N4 windowed substrates. It was found that the microstructure, crystallography and magnetic properties varied a great deal depending on which substrate the films were deposited onto.