The influence of sodium hydroxide, water density and reaction temperature on the hydrothermal gasification of a compositionally complex municipal solid waste material in the form of refuse derived fuel (RDF) has been studied. RDF and sodium hydroxide corresponding to NaOH/C mass ratios of 0.2 to 1.2 were reacted at 375 °C using initial water densities of 0.04, 0.10, and 0.15 g · cm-3 in a 500 mL stainless steel batch reactor. The effect of reaction temperature was studied between 300 and 375 °C. The reactions produced a hydrogen-rich gaseous product. Other gases included CO, CO2, and small amounts of C1-C4 hydrocarbons. The results showed that hydrogen gas production increased dramatically with increasing NaOH/C mass ratio up to a ratio of 0.8 but decreased slightly when the ratio was increased to 1.2. CO2 and CO concentrations decreased with increasing NaOH/C mass ratio, indicating that NaOH catalyzed the gasification via removal of CO2 as carbonate. Hydrogen production was also enhanced with increasing water density up to the reactor design limit. The results further suggest that the predominant reactions taking place during this process can be represented as 2C(s) + 2NaOH(aq) + 3H2O(g) → Na2CO3(aq) + 4H2(g) + CO2(g). Comparable gasification results were obtained for glucose, cellulose, and starch under a set of identical reaction conditions.