To provide a basis for strategies to prevent the formation of potentially explosive gas mixtures during Bayer digestion and wet oxidation processes, a quantitative description of the hydrogen production capacities of industrial liquor is required, along with an improved understanding of the reactions that produce hydrogen gas. This study is the fourth in a series aimed at addressing these knowledge gaps. For a low-temperature refinery liquor, it was found that doubling the TOC conversion from 20 to 40% resulted in a 5-fold increase in the amount of hydrogen evolved (similar to 0.06 mol H-2/L of liquor), indicating that the degradation of refractory organic compounds in the liquor contributes to a significant amount of the hydrogen produced. Critically, more hydrogen was produced under wet oxidation conditions than under the largely anoxic conditions encountered in digestion, with the magnitude of this effect diminishing with an increase in temperature (180-270 degrees C). The wet oxidation of several model organic compounds representing various structural classes known to be present in the liquor was also investigated. With oxygen addition, aliphatic compounds (3-hydroxybutanoic acid, maleic acid, and erythritol) produced less hydrogen compared with the same :reactions conducted under anoxic conditions; conversely, aromatic compounds (benzoic acid, In-salicylic acid, gallic acid, and catechol) produced more hydrogen in the presence of oxygen, which was attributed to benzene ring-opening reactions generating unsaturated intermediates which are known hydrogen producers. The effect of oxygen On hydrogen production rates is therefore expected to be largely governed by the relative and total amounts of aromatic and aliphatic carbon in the liquor. Because the formation of hydrogen from complex mixtures of low molecular weight (LMW) organic compounds often occurs through short-lived aldehyde intermediates, the reactions of acetaldehyde, butyraldehyde; benzaldehyde, and glyoxylate were studied under similar conditions. A high-temperature pathway for the quantitative conversion of aldehydes to hydrogen and their corresponding carboxylic acids was identified in the case where the aldehydes were injected at low concentration into a preheated sodium hydroxide, solutio (<= 1 mM aldehyde, 3 M NaOH, 250 degrees C, N-2).