Effects of adsorption of potassium iodide (KI) on hydrogen content of the plate-type Raney nickel catalyst and its catalytic activity for hydrogenation of alcohol and ketones were investigated. The amounts of K⁺ and I^- were determined by potentiometry and radioactivity measurement. The adsorption isotherms in lower (∼5.5×10^-5mol dm^-3) and higher KI concentration ranges were represented by Freundlich's and BET equations, respectively. The residual hydrogen content of the catalyst and hydrogenation activities of ketones decreased exponentially with an increase in the amount of I^- adsorbed. The surface coverage causing the hydrogenation activity to decrease by 50% was 3% for allyl alcohol and 0.3% for 2-butanone. With respect to allyl alcohol, even such a catalyst as had preadsorbed ca. 5 monolayer amounts of I^-possessed a 33% residual activity.