Hydrothermolysis of D-glucose, cellobiose, and beta-cyclodextrin leads to the formation of small amounts of 3-deoxyhexonic acids. These acids are known to be produced by the alkaline degradation of carbohydrates. The formation under neutral hydrothermal conditions of the 3-deoxyhexonic acids provides evidence for the formation of 3-deoxy-D-erythro-hex-2-ulose, a compound that has been reported to play a role in both alkaline and acidic conversion of carbohydrates. Hydrothermolysis of 2- and 3-deoxy-D-arabino-hexonic acid does not lead to significant decarboxylation, and therefore these compounds cannot be considered to play a major role in the initial hydrothermal decarboxylation of biomass.