Rates of hydrolysis of O,S-diethyl phenylphonsphonothioate (DEPP) and p-nitrophenyl diethyl phosphate (Paraoxon) in 0.1 M aqueous NaOH dispersions of a cross-linked poly(2-ethylhexyl methacrylate) latex containing styrylmethyl(trimethyl)ammonium chloride units were measured by P-31 NMR spectroscopy. The reactions followed second-order kinetics to 75% conversion. The rate constants of hydrolysis of both DEPP and Paraoxon were up to six times faster than those in the absence of the latex. Hydrolysis of DEPP gave a 85/15 ratio of products from P-S versus P-O bond breaking in the absence of latex and a 90/10 ratio in the presence of latex. P-31 NMR relaxation times and visual observations show that DEPP, Paraoxon, the products of DEPP hydrolysis, and p-nitrophenoxide ion all partition from water into the latex. The diethyl phosphate ion that is produced from Paraoxon partitions into water. The kinetics at these high concentrations of DEPP and Paraoxon do not fit the enzymelike and ion exchange models that have been applied to the kinetics of reactions of lower concentrations of substrates in latex dispersions.