AbstractPartition coefficients in poly(ethylene glycol)/dextran aqueous two‐phase systems are reported for mixed‐casein and its components, α, β and κ casein. Rates of casein proteolysis by α‐chymotrypsin and by trypsin are reported in single‐phase and aqueous two‐phase reactor systems. The advantages resulting from selective partitioning of substrates, enzymes, and products are examined in terms of relative volumetric reaction rates.