Phase transfer catalysis has been in use for over three decades, but triphase catalysis, where the phase transfer catalyst is immobilized on a solid support, is of much more recent origin. Polymeric solids are among the most commonly used supports, but they lead to diffusion-reaction problems considerably more complicated than in conventional solid catalyzed reactions. Since, rather unaccountably, triphase catalysis has not been a subject of common interest in chemical engineering, a brief review of the field is given with emphasis on the role of catalyst synthesis in understanding its diffusional implications. This is followed by the development of a mathematical model for analyzing the diffusion-reaction problem in a common triphase catalytic system : liquid-liquid-solid triphase catalysis.