Cross-linked polystyrene latexes containing (styrylmethyl)trialkylammonium chloride repeat units greatly accelerate the o-iodosobenzoate (IBA) catalyzed hydrolysis of p-nitrophenyl diphenyl phosphate (PNPDPP). The half-life of PNPDPP is as short as 3 s with 0.10 mg mL(-1) of particles containing 24 mol % of (styrylmethyl)tributylammonium chloride repeat units (TBAQ24) and 4 x 10(-5) M IBA in TAPS buffer at pH 9.1 and 25.0 degrees C, and the apparent second-order rate constant (k(IBA)) exceeds that in the absence of latexes by 6300 times, the largest rate enhancement reported for IBA-catalyzed hydrolysis of PNPDPP in any colloidal or polymeric medium. The k(IBA) value increases with increasing radius of the quaternary ammonium ion (trimethyl < triethyl < tripropyl < tributyl) and with decreasing mole % of (styrylmethyl)trimethylammonium chloride (TMAQ) repeat units in the latexes. The amounts of IBA and chloride ions bound to latexes were measured directly by UV spectrophotometry and chloride-selective electrode analysis of ultrafiltrates from dispersions, and ion exchange selectivity coefficients for IBA and TAPS anions relative to chloride were calculated. The sizes of water-swollen particles were measured by dynamic and static light scattering. Using an ion-exchange model, the second-order rate constants k(2L) in the latex phase were estimated from observed rate constants, the percent of IBA bound, and the particle volumes. The 6300 times rate enhancement in the TBAQ24 dispersion is due ten times to a higher intraparticle rate constant and 630 times to higher local concentrations of IBA and PNPDPP in the latex phase. With the most hydrophilic TMAQ latex particles, the rate enhancements are due entirely to higher reactant concentrations in the particles. The intraparticle rate constants at pH 9.1 and the ion exchange selectivity coefficients can be used to predict rate constants as functions of pH, amount of latex particles, and amount of added NaCl.