The deformation process in rubber-modified polystyrene was studied by static SAXS on deformed tensile samples under loaded conditions. The typical scattering pattern of these systems was interpreted as originating from two components : crazing in the polystyrene matrix and a dominant cavitational component associated with the rubbery phase. We have used and checked a new method to separate and quantify the crazing component and to give an estimation of the noncrazing component. The contribution of matrix crazing to the plastic deformation is of minor importance. With an increasing content of rubber, the amount of crazing decreases strongly whereas the (impact) toughness increases. In both PS/SEBS blends and HIPS the main source of plastic energy absorption might be cavitation-induced microscopic shear yielding.