Swelling and dissolution behavior of poly(methyl methacrylate) (PMMA) films during unidimensional penetration of methyl ethyl ketone/methyl alcohol (MEK/MA) liquid mixtures at 22 C is presented. Optical microscopy and two-beam interferometry were applied to clamped PMMA films to obtain information on penetration kinetics and penetrant concentration profiles. Dissolution by pure MEK was initially controlled by Case II penetration kinetics and at later stages of the process, by stress cracking in the absence of a surface layer. Introduction of increasing amounts of MA in the liquid solvent resulted in moderation of the fragmentation process, enhanced penetration rates at the early stages of the process, deviations from Case II kinetics at the later stages, and the existence of a surface layer. These results indicate that penetration of MEK/MA mixtures and dissolution of PMMA are characterized by lower diffusion Deborah, and higher dissolution, numbers compared to those of pure MEK. Swelling by pure MA, as well as by nonsolvent MEK/MA mixtures, was characterized by Case II penetration kinetics. A pronounced minimum in the penetration rate versus liquid mixture composition plot was observed at 30 : 70 v/v MEK/MA composition. Laser interferometry was applied to very thin PMMA films, supported on a silicon substrate, to study selected cases. The results obtained were very similar to those obtained by optical microscopy.