Transmission electron microscopy (TEM) has been used to investigate the microdeformation mechanisms in thin films (< 1 mu m) of MMA-glutarimide copolymers of various compositions ranging from 36 to 76 mol% glutarimide. These polymers all have an entanglement density of similar to 5 x 10(25) m(-3), which is of the same order as that in poly(methyl methacrylate) (PMMA). Crazing is known to be the dominant deformation mechanism in thin films of PMMA tested at room temperature. As the glutarimide content is increased in the copolymers, however, motions associated with formation of shear deformation zones (DZs) are facilitated, and these become increasingly competitive with scission crazing in thin films. The presence of the MMA groups may nevertheless continue to hinder large-scale chain motions associated with high-temperature disentanglement crazing, since it was found that the absolute temperature for the onset of disentanglement in the copolymers remained similar to that in PMMA (ca. 80 degrees C for the strain rates used here), and was essentially independent of the composition and the glass transition temperature (T-g).