Composition dependences of the glass transition temperature (T g ), the molecular weight between entanglements (M e ) and the thin film deformation behaviour have been studied in two series of methyl methacrylate-based random copolymers. When the comonomer is styrene, increases in both M e and brittle behaviour accompany the decrease of T g . When the comonomer is N-substituted maleimide, the same conclusions hold with increasing T g . For both series of materials, a correlation is proposed between the number of skeletal bonds in an entanglement strand, N e , and the quantity (T g --T c s ), whereT c s is the lower temperature at which shear deformation is observed in thin films in competition with crazing.