Reactions of different carbonaceous materials in alkaline hydrothermal media have been studied in a batch reactor. In general, hydrogen gas production increased in the presence of sodium hydroxide for all samples. Based on experimental evidence, a chemical reaction model was developed for hydrogen gas production from these reactions. Evaluation of gaseous reaction products suggested that two main reaction types could have occurred with the samples. These were alkaline oxidation (degradation) leading to hydrogen gas and methane production, and thermal degradation (pyrolysis) leading to the production of tars and hydrocarbon gases. The carbohydrate-rich samples including glycerol, xylan and lignin produced more hydrogen gas via alkaline oxidation than coal, waste tyre and phenol. Pyrolysis was found to be the predominant reaction for coal and waste tyre. In the presence of NaOH, glycerol produced the highest hydrogen gas yield of 76%, while waste tyre produced the lowest yield of 8.2%. The proportions of hydrogen gas and methane in the gas product could be linked to the role of the stoichiometric reacting water during and after alkaline degradation. In addition, phenol was highly resistant to thermal degradation but less so to alkaline degradation. Results suggest that the chemical nature of the carbon atoms in the samples was crucial in influencing the dominant reaction type.