Abstract The aquation of tris(methylmalonato)chromate(III) anion was studied at 25–40° in 0·10–1·0 F HClO 4 (μ = 0·20−1·0, NaClO 4). The observed pseudo-first order rate constant k obs was found to be dependent on the hydrogen-ion concentration over the entire range of [H +]: k obs = k′[H +] + k″ . At 25° k′ = 5·88 × 10 −41mole −1sec −1 , and k″ = 0·967 × 10 −4sec −1 . Over the temperature range E a = 22·0kcal/mol, and log PZ = 16·5. The product of aquation was found to be trans-[Cr(Mmal) 2(H 2O) 2] − (Mmal = methylmalonate), and a mechanism in which a dissociation of the protonated intermediate, [Cr(Mmal) 2(HMmal)] 2−, followed by a fast attack of two water molecules in opposite directions is proposed.