Flow stress, Young's Modulus, energy and strain of fracture of poly(methyl methacrylate) (PMMA) and polystyrene ( PS) were studied under compressive loading at strain rates of 10(-4)-10 s(-1) and temperatures from 293 K to temperatures similar to 20 K below T-g. It was found that the energy of fracture shows an increase in the quasi-static strain rate (10(-4)-10(-3) s(-1)) region and becomes constant in the low strain rate (10(-2)-10 s(-1)) region, while the strain of fracture shows a slow decrease with rate over the strain rate range tested. The activation energies and volumes of PMMA and PS at yield stress, 20% and 30% strain were evaluated using Eyring's theory of viscous flow. Delta G was found to be constant for all strain rates and strains for both PMMA and PS. The activation volume for both materials increased as a function of strain.