In attempts to improve the properties of post-consumer commingled plastics waste a variety of additives that could potentially act as compatibilizers/impact modifiers were evaluated. The feedstock was representative of a model curbside collection program and contained a variety of polymers, mostly polyethylenes with PET, PP, PVC, etc., in smaller amounts. The ground mixed plastics were first compounded and melt filtered in a counter-rotating nonintermeshing twin-screw extruder and then combined with different amounts of additives in a corotating intermeshing twin-screw extruder. Additives included unmodified and maleated polyolefin elastomers and styrene/olefin block copolymers. Blends were analyzed for thermal and mechanical properties, and processability. The most effective modifier in terms of impact strength improvement was a styrenic block copolymer with very similar rheological characteristics to the commingled plastics matrix. The experimental observations were interpreted by considering the complex morphological features of the injection molded multicomponent, multiphase systems.