The kinetics of aqueous-phase hydrogenation of lactic acid to propylene glycol over a 5 wt % Ru/carbon catalyst have been characterized in a stirred batch reactor. A thorough analysis of mass-transfer resistances based on measurements of hydrogen solubility and gas-liquid masstransfer coefficients, application of correlations in the literature, and intraparticle diffusion calculations show that mass-transfer resistances are negligible at the temperatures (403-423 K) and hydrogen pressures (6.8-1-3.6 MPa) studied, A Langmuir-Hinshelwood (L-H) model is proposed and used to fit lactic acid conversion kinetics. The kinetic model provides insight into the catalytic reaction mechanism and forms the basis for design and further investigation of the aqueous-phase hydrogenation.