The thermal decompositions of cellobiose, maltose, dextrose, and potato starch have been studied over a temperature range, by following the production of volatile products. Carbon dioxide, carbon monoxide, and water with small quantities of acids, aldehydes, and volatile solids were produced in all cases. With cellobiose, the first step of the reaction, which involved the elimination of two moles of water per mole of sugar, could be separated from the second step, where the oxides of carbon were produced, by controlling the reaction temperature. Dextrose first dimerized by a rapid reaction and then decomposed in much the same manner as cellobiose. The behavior of maltose was anomalous and no dehydration by a separate step could be detected. The decomposition of potato starch was similar to the second step of the cellobiose reaction.