Using atomic force microscopy, film stability is investigated for poly(methyl methacrylate) (PMMA)/poly(styrene-ran-acrylonitrile) (SAN) thin film blends ranging from 10 nm to a few microns thick and deposited on an oxide covered silicon substrate. In addition to characterizing the surface morphology, the PMMA rich phase which wets the surface and oxide, is selectively etched to reveal the underlying or interfacial phase morphology. For 50/50 blends at 158 degreesC, films are found to be stable for thickness values between 20 and 100 run. Dewetting and phase separation mechanisms are invoked to understand why films are unstable below and above this range, respectively. By annealing 25/75, 50/50 and 75/25 thin film blends between 158 and 200 degreesC, a stability diagram is constructed and shows that blends with low PMMA volume fractions (i.e. wetting component) are more stable than blends rich in PMMA. In some cases, phase separation is observed at temperatures as low as 158 degreesC, which is about 12 degreesC below the bulk lower critical solution temperature (LCST). Film stability is analyzed by predicting the PMMA concentration necessary to produce a surface wetting layer at its equilibrium thickness.