The kinetics of aqueous phase catalytic hydrogenation of maleic acid to succinic acid were studied in a slurry reactor between 323 and 353 K using a hydrogen partial pressure range of 1.03-2.06 MPa, with a 5% Ru/Al2O3 catalyst. The initial rate data were analyzed using a power law model. Above 1.72 MPa hydrogen partial pressure, the reaction was zero order with respect to hydrogen. Based on this hydrogen pressure effect, a Langmuir-Hinshelwood rate model was proposed to which the rate data could be fitted well. (C) 2003 Society of Chemical Industry.