The conversion of lignocellulosic biomass into value-added chemicals using non-toxic heterogeneous catalysts and water as solvent is an attractive green process. Biomass-derived lactic acid is an important renewable chemical building block for synthesizing commodity chemicals, e.g. biodegradable plastics. This paper reports that hemicellulosic biomass, xylan and xylose, can be converted to lactic acid over a ZrO2 catalyst starting from pH neutral aqueous solutions. The effects of reaction conditions, including temperature, oxygen partial pressure, biomass loading, and catalyst loading, etc., on the conversions of hemicellulosic biomass and the corresponding yields of lactic acid have been investigated. Molar yields of lactic acid, up to 42% and 30% were produced from xylose and xylan, respectively, under the investigated reaction conditions and with the ZrO2 catalyst. The key intermediates such as glyceraldehyde, glycolaldehyde and pyruvaldehyde were used as the reactants to probe the reaction mechanism. The role of the ZrO2 catalyst in the retro-aldol condensation of xylose, as well as the catalyst stability, has been discussed. (C) 2014 Elsevier B.V. All rights reserved.