Utilization of biomass and waste products for the production of synthesis gas is an important step towards generating useful fuels from renewable resources. Cellulose and glycerin are two such products that can either be decomposed into useful chemicals or completely gasified. One technique that is used is the decomposition of these compounds in a supercritical water medium. To use this process effectively, a complete and comprehensive understanding of the reaction pathway and the kinetics of decomposition is required. Two of the intermediate compounds produced during the decomposition of cellulose and glycerin in supercritical water are formaldehyde and acetaldehyde. This study was performed to elucidate the various reactions of formaldehyde and acetaldehyde in supercritical water. Experiments were conducted non-catalytically in a continuous, 0.1 L Haynes 282 alloy reactor at 24.2 MPa with a residence time of 30 s. Temperatures of 500 to 700 °C were investigated at water-to-carbon molar feed ratios of 5:1 and 3.8:1 for formaldehyde and acetaldehyde respectively. By using the analysis results for the gas and liquid effluents, reaction schemes were constructed that described the reaction behavior of formaldehyde and acetaldehyde in supercritical water under gasification conditions.