Separation of poly(ethylene terephthalate) (PET) from poly[vinyl chloride) (PVC] is an important problem when recycling bottles and other consumer products. Previous work has identified a processing window for the selective grinding of a mixture of PET and PVC. Impact grinding for conditions within the processing window will ensure that one polymer fails in the ductile mode and the other fails in the brittle mode, giving differences in sizes and shapes between the ground polymers, The ground mixture can be Separated by physical means such as screening. This study develops size distribution models for the impact grinding of PET and PVC chips, an important tool for the operation and control of a selective grinding process. The distribution coefficients can be related to grinding conditions, and to the failure mechanism of the polymer.