The equilibrium of 1,3-dihydroxy-2-propanone (dihydroxyacetone) and 2,3-dihydroxypropanal (glyceraldehyde) and their dehydration reaction to methylglyoxal has been studied in the range of 180 - 240 °C under hydrothermal reaction conditions. A reaction mechanism for the conversion of these C-3 compounds was formulated which allowed the calculation of the product yields. In the investigated temperature range both compounds produce a maximum of 30-40% of methylglyoxal.