BACKGROUND: Currently, the 'green chemistry' philosophy is being increasingly adopted by the chemical industry and, therefore, new production procedures of valuable chemicals from biomass-derived raw materials are being sought. In this work, the synthesis of lactic acid from sorbitol under alkaline hydrothermal conditions is investigated by analyzing the influence on conversions and yields of temperature, NaOH/sorbitol molar ratio (MR), initial sorbitol concentration (SC) and reaction time. RESULTS: A 100% sorbitol conversion and a maximum 39.5% yield of lactic acid on a carbon basis are obtained at 280 degrees C, 50 min, 1.0 mol L(-1) SC and 2.0 MR. Glyceraldehyde was the only identified intermediate while formic acid, acrylic acid, acetic acid, oxalic acid and sodium carbonate were identified as over-oxidation products, all of them in very low yields with the exception of formic acid (16% yield at a MR of 4 and 280 degrees C). Several plausible conversion routes of sorbitol involving dehydrations, keto-enol tautomerisms, reverse aldol condensations, aldol condensations, Cannizzaro reactions and oxidations are proposed. CONCLUSIONS: Considering the high number of parallel conversion routes as a consequence of high functionality of sorbitol, the 39.5% lactic acid yield obtained is a good result. Total carbon mass in all identified products only justifies, at most, 50% of that in sorbitol due to the coexistence of several conversion routes resulting in a large number of products other than lactic acid. (C) 2011 Society of Chemical Industry