Polymer latexes promote hydrolyses and oxidations of organic compounds in aqueous dispersions by concentrating reactants and catalysts into the small volume of the latex phase, and by increasing the intrinsic rate constants. Monodisperse styrene copolymer latexes were prepared with 0-75 wt% chloromethylstyrenes and 1 wt% divinylbenzene to crosslink the polymer particles. Treatments of the copolymers with tertiary amines produced latexes containing widely varied structures and 1-60 mol% of quaternary ammonium ions (1), which confer different degrees of swelling and hydrophile-lipophile balance to the particles. Kinetics of two reactions that proceed in quantitative yield were studied to determine the contributions to overall rate accelerations of intraparticle rate constants and partitioning of reactants and catalysts between the particle and aqueous phases. The intraparticle rate constant for unimolecular decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion (2) is 310 times faster in a hydrophilic (styrylmethyl)trimethylammonium ion exchange latex than in water and 21,000 times faster in a more lipophilic polystyrene latex containing (styrylmethyl)tributylammonium ion exchange sites (1). This is the fastest rate of decarboxylation of 2 ever observed in an aqueous medium. A dendrimer with 36 alkyl(trimethyl)ammonium ion chain ends accelerates decarboxylation of 2, but is much less active than CTACl micelles and cationic latexes. Aqueous dispersions containing 0.1-0.8 mg/ml of monodisperse (styrylmethyl)trimethylammonium latexes and 5.0 x 10(-5) M o-iodosobenzoate ion (IBA, 3) catalyze hydrolysis of p-nitrophenyl diphenyl phosphate (PNPDPP, 4) at 25 degrees C at pH 8.0 with half lives as short as 10 s, the same as the previous fastest rate obtained with IBA in CTACl (cetyltrimethylammonium chloride) micelles. The weight concentration of latex particles needed for optimal catalysis is less than the weight of CTACl at its critical micelle concentration. Analysis of the results using an ion-exchange model proves that the highest rate with the (styrylmethyl)tributylammonium latex relative to the rate in water, is due about 10-fold to a higher intraparticle rate constant, and 600-fold to higher concentrations of PNPDPP and IBA in the particles. Since dendrimers and latexes containing only surface quaternary ammonium ion exchange sites are much less active than latexes having quaternary ammonium ions in the more lipophilic core, catalysis of these reactions proceeds mainly in, not on, polymer latexes.