Efficient cleavage of the beta-O-4 linkages in lignin is the key to obtaining phenolic chemicals from lignin. The hydrothermal treatment of lignin in neutral and alkaline water is highly utilized but remains poorly understood, preventing the development of strategic depolymerization methods. Herein, the hydrothermal decomposition of a guaiacol-based beta-O-4 lignin dimer is systematically explored at varying hydroxide and carbonate base concentrations at 175 degrees C and supplemented by density functional theory calculations to elucidate the mechanisms of monomeric phenolic product formation. Weakly basic conditions were found to efficiently cleave the beta-O-4 linkage yielding guaiacol as the primary product and vanillin and acetovanillone as minor products. Intramolecular substitution, bimolecular elimination, and intermolecular substitution mechanisms are proposed for beta-O-4 bond cleavage, and their theoretical rates are compared by transition state theory calculations. The downstream product pathways of this reaction are also analyzed and are the first to account for vanillin production under these conditions. Major beta-O-4 bond cleavage and dimer decomposition paths have been identified. In neutral water, the dominant path is quinone methide formation followed by homolysis. In basic water, two paths dominate: one leads to a stable vinyl ether, and the other leads to a highly unstable homovanillin.