Poly(methyl methacrylate) PMMA is used extensively as an electron beam resist. The development of the lithographic pattern after exposure of the polymer to electron beam irradiation is achieved by contacting the film with a solvent mixture of isopropyl alcohol and methyl ethyl ketone. In this paper the mutual diffusion coefficients for isopropyl alcohol-methyl ethyl ketone mixed solvents into PMMA of narrow molar mass distribution with a range from 49000 to 400000 g mol ^–1 are reported. The mutual diffusion coefficient indicates that the behaviour was both a function of molar mass and solvent composition. The ability for the PMMA to swell in the solvent was found to be molar mass dependent. Cloud point measurements on solutions indicated that change in the molar mass influenced the temperature at which precipitation occurred in the mixed solvents. The cloud points also depended on the quality of the solvent; occurring at lower temperature for the better solvent [1:1 w/w isopropyl alcohol (IPA)-methyl ethyl ketone (MEK)] and being higher for the poorer solvent (4:1 w/w IPA-MEK). The swelling characteristics indicate that an increase in the degree of chain entanglement leads to suppression of the degree of swelling. The electron beam sensitivity suggests a complex interplay of the effects of the molar mass on the chain scission process, on the ability of solvent to enter the degraded matrix and on the solubility of the polymer on the solvent mixture. The casting solvent has an effect on the structure of the solid and influences the behaviour of the films when exposed to the developing solvent.