Hydrothermal transformations of organic compounds play important roles in subsurface carbon cycling processes that constrain planetary habitability. Hydrothermal reactions of organic compounds can be influenced by the coexisting inorganic composition, as water-rock interactions release dissolved metals that may impact organic reactivity. Carboxylic acids are abundant in organic-rich hydrothermal environments, and decarboxylation is a major reaction path that produces new organic species (e.g., alkanes). However, studies of hydrothermal interactions between carboxylic acids and inorganic species are few, and in particular, the effect of dissolved metal ions on acid decarboxylation is largely unknown. Here, we consider phenylacetic acid (PAA) as a model carboxylic acid compound to study its hydrothermal decarboxylation in the presence of copper (II) salts. At 200 ºC and 15 bar (Psat), PAA was nearly unreactive in pure water, whereas up to 40% was decomposed with copper (II) sulfate/chloride/acetate after 2 hours, and more than 70% was degraded with copper (II) nitrate. Time series and PAA substituents experiments indicated an oxidative decarboxylation pathway from PAA to benzyl alcohol, benzaldehyde, and then benzoic acid. Results also showed that benzoic acid/benzaldehyde ratios ranged between 0.3 and 2.3 in the cupric sulfate/chloride/acetate experiments, whereas in the presence of cupric nitrate the ratio increased to 47.6 after 6 hours, suggesting a much stronger oxidizing effect of cupric nitrate. Additional experiments with other divalent metal ions (e.g., Mg, Ni, Zn) did not show similar oxidizing effects as copper. Raman spectroscopy analysis further suggested that the hydrothermal oxidative decarboxylation could be attributed to the formation of Cu-coordinated complexes. Our results highlight a potentially significant role of dissolved copper in hydrothermal organic redox transformations, which could provide new insights into understanding water-rock-organic interactions in natural and artificial hydrothermal systems.