The synthesis of lactic acid by alkaline hydrothermal conversion of glycerol at high glycerol concentrations under autogenous pressure is described. Temperature (250-290 degrees C), NaOH/glycerol molar ratio (1.1-1.75), glycerol concentration (2.5-3.5 M), and reaction time (30-250 min) have been the variables studied with temperature and NaOH/glycerol molar ratio having the major influence. An 84.5% lactic acid (as sodium lactate) yield is obtained at 280 degrees C and 90 min with a 2.5 M glycerol concentration and a 1.1 NaOH/glycerol molar ratio. The same yield is obtained starting from crude glycerol from biodiesel production as raw material. Sodium acrylate, oxalate and formate, and sodium carbonate have been identified as the main byproducts with carbonate being the major one. Several conversion routes based on the experimental results are proposed. A process for lactic acid separation based on electrodialysis is outlined.