For given nominal molar concentrations in complementary functional groups, the common wisdom is that the reaction rate of those attached to large molecules does not exceed that of small molecule analogues. They are at best equal. The difference in reaction kinetics is more pronounced if reactive molecules are so large that they phase-separate, which is often so for polymers. In such cases, complementary functional groups from the two phases can meet and react only at the interfaces. Thus, reaction kinetics of functional groups attached to small molecules has always been taken as the upper bound for those attached to polymers. Here we show that under sufficient mixing, reaction kinetics of two immiscible reactive polymers can be significantly higher than that of small molecule analogues. This phenomenon is attributed to the combined effects of increasing collision probability originated from mixing-induced interfacial area generation, longer relaxation time of reactive polymer chains, and eventually interfacial slip. (C) 2004 American Institute of Chemical Engineers.