The neutral rhodium(iii) mer-[Rh(C2O4)Cl(py)3] (1) complex was found to release oxalate in concentrated aqueous solutions of strong acids (HNO3, HClO4, and CF3SO3H), generating the heteroleptic cis-diaquacomplex cis,mer-[RhCl(H2O)2(py)3]2+ (3). The process proceeded via formation of intermediate complex 2 ([Rh(HC2O4)Cl(H2O)(py)3]+) with the monodentately coordinated oxalato ligand. The structure of the intermediate complex 2 was probed using density functional theory (DFT) calculations. Experimental 13C nuclear magnetic resonance (NMR) data on the kinetics of oxalate release at 25 °C were evaluated within a consecutive reaction model with two pseudo-first order stages. The logarithms of the rate constants linearly depended on the Hammett acidity function of the medium. Cationic cis-diaquacomplex 3 was found to act as an active water reduction catalyst (WRC) under visible light irradiation when paired with the [Ir(ppy)2bpy]+ photosensitizer (PS).