Hydrothermal reactions (oxidation and hydrolysis) involving lactic acid (LA) were studied at temperatures ranging from 300 to 400 degrees C and a nominal pressure of 27.6 MPa. Kinetic models were developed with respect to concentrations of LA and total organic carbon (TOC), respectively. The best-fit model for LA oxidation with 95% confidence limits is -d[LA]/dt = 10(18.7+/-4.2) X exp(-226 +/- 46.6 kJ/mol/RT)[LA](0.88+/-0.11)[O-2](0.16+/-0.19). Similarly, the best-fit TOC model for lactic acid oxidation is -d[TOC]/dt = 10(4.3+/-2.5) exp(-68.4 +/- 27.2 kJ/mol/RT)[TOC] (0.62+/-0.33)[O-2](0.36+/-0.26) The best-fit TOC model for lactic acid hydrolysis is -d[TOC]/dt = 10(8.4+/-2.1) exp(-125 +/- 26.7 kJ/mol/RT)[TOC]. On the basis of identified liquid and gaseous products, pathways for hydrothermal reactions involving lactic acid were proposed. Acetic acid and acetaldehyde were confirmed as the major liquid intermediates for oxidation and hydrolysis reactions, respectively. Carbon monoxide and methane were identified as the major gaseous byproducts from these reactions. These results demonstrated the potential of completely oxidizing, as well as converting, lactic acid into other organic products, in high-temperature water.