The reactions of meso-tetrakis( p-sulfonatophenyl)porphinatodiaquorhodate(III) [Rh(TPPS)(H 2O) 2] 3− and meso-tetrakis(4- N-methylpyridyl)porphinediaquocobaltate(III), [Co(TMPP)(H 2O) 2] 5+ with thiourea have been studied in 0.1 M H + and μ = 1.0 M (NaNO 3) at different temperatures and pressures. The kinetic parameters for the substitution of the first aquo ligand by thiourea are as follows for [Rh(TPPS)(H 2O) 2] 3− and [Co(TMPP)(H 2O) 2] 5+ respectively: k 1 at 25 °C; 4.69 × 10 −2 and 8.88 × 10 −1 M −1 s −1, Δ H ≠; 85.1±1.3 and 91.4±0.7 kJ mol −1, Δ S ≠; 14.9±4.2 and 60.6±2.2 J K −1 mol −1, † V ≠; 11.1±0.5 and 12.6±0.6 cm 3 mol −1. These results were interpreted as evidence for a dissociative mechanism.