The kinetics of the reaction between gaseous ethylene oxide and semidry granular potato starch was studied in a pressure-controlled semibatch reactor with and without impregnation of the starch with the catalyst sodium hydroxide. Four parallel reactions are involved : the catalyzed (with reaction rate R(RO)-) and uncatalyzed hydroxyethylation of starch (rate R(ROH)) and the catalyzed (rate R(OH)-) and uncatalyzed hydrolysis of ethylene oxide (rate R(H2O)). For 313 < temperature T < 368 K, 9.9 < moisture content W < 22.5 wt % dry basis, 0 < catalyst concentration C-NaOH < 40 mmol/mol of anhydroglucose units, and 0 < ethylene oxide partial pressure p(EO) < 220 kPa, the reaction rates in units could be fitted by the following : uncatalyzed hydroxyethylation, eq 9; uncatalyzed hydrolysis, eq 10; catalyzed hydroxyethylation, eq 11; catalyzed hydrolysis, eq 12. The activity a(EO) = p(EO)/p(EO)(0) and p(EO)(0) is the saturation pressure of ethylene oxide. High selectivities, approaching 80% toward the desired product hydroxyethyl starch, could be obtained at high T, low W, and high C-NaOH.