The formation of diketopiperazines from the dipeptides isoleucylglycine and glycylisoleucine was investigated at 132 degrees C over the pH range approximately 2 to 10. At pH 6.2, approximately 50 percent of the original dipeptides were converted to the diketopiperazines during the heating experiments. Hydrolysis of the diketopiperazines can yield either the original dipetide or an inverted dipeptide product. The isoleucine in the diketopiperazines was the most highly epimerized component in the system. Previous racemization and epimerization studies with dipeptides have not taken into account the formation of diketopiperazines and, as a result, the cortclusions about the mechanism and geochemical implications of amino acid racemization in dipeptides will require revision.