Hydrolysis of soybean protein to amino acids in a hydrothermal batch reactor was conducted. The effects of the initial raw material concentration, reaction temperature, and holding time on the yield of amino acids were investigated. Based on the results of a soybean protein hydrothermolysis optimization analysis, a mathematical model for the reaction was developed. The highest amino acid production was observed for suspensions containing no more than 10 wt % soybean protein. At this concentration, in good agreement with the experimental results, a maximum estimated amino acid yield of 0.63 g/g of raw material was obtained at 250 degrees C after 14 min. Secondary reactions involving decarboxylation, deamination, transamination, and oxidation might explain the different amino acid profiles before and after hydrothermolysis.