Polymer pollution from nonbiodegradable polymer blends is a source of increasing environmental concern. The obvious solution, the replacement of petroleum-based polymers with biodegradable polymers from renewable sources, is difficult, since these polymers are highly immiscible. Here, we show how the addition of only a few percent of a tertiary phase can enable compatibilization and enhancement of the mechanical properties. The tertiary phase can be selected for any binary polymer system using the spreading coefficient (lambda(ij)) and the work of adhesion (W-a) as guides. We demonstrate this method using poly(lactic acid) as the continuous matrix phase, styrenic polymers, polystyrene (PS) or high-impact polystyrene (HIPS), as the discontinuous minority phase, and styrene acrylic copolymers [styrene methyl methacrylate (SMMA)] as the tertiary phase. We show that a distinct maximum in the impact toughness, nearly 58 and 56% for PS and HIPS blends, respectively, occurs when only 1.2 and 1.5% SMMA copolymers are added.