A two-phase kinetic model is presented for the base-catalyzed hydroxyethylation of potato starch using ethylene oxide at temperatures between 293 and 318 K in aqueous starch slurries containing sodium sulfate. The rate of the hydroxyethylation of starch as a function of starch anion concentration (c(RO,s)), ethylene oxide concentration in the aqueous phase (c(EO)), and temperature is described by : R(HES) = 0.958 x 10(9) exp(-74680/RT)c(EO)c(RO,s) kmol M-3 of starch s-1. The observed kinetics of an important side reaction, the hydrolysis of ethylene oxide to ethylene glycol, is in accordance with literature data. In the starch phase the hydrolysis rate is considerably smaller than that in the continuous water phase. In aqueous salt solutions the rate constants k(H2O) and k(OH) of the hydrolysis of ethylene oxide are enhanced by 46% and reduced by 7% per kmol/m3 of ionic strength, respectively. The hydroxyethylation of glycol follows the rate equation (in the absence of starch and salt) : R(DEG) = 50.2 exp(-34500/RT)C(EO)0.62c(EG)0.45c(OH)0.95 kmol M-3 s-1. The hydroxyethylation of sulfate proceeds according to : R(HHS) = 8.48 X 10(8) exp(-86000/RT)c(EO)c(SO4).