The reactions of cis- and trans-diaquabis(methylmalonato)chromate(III) anions in 0·2–1·0 F HClO 4 (μ = 1·0, NaClO 4) were studied at 25–40°. From the chromatographic results, it was found that both reactions were aquation to tetraaqua(methylmalonato)chromium(III) cation. The pseudo-first order rate constant k obs for both reactions was found to follow the equation k obs = k′[H +] = k″. At 40°, k′ = 1·76 × 10 −4 l mol −1 sec −1 and k″ = 0·130 × 10 −4 sec −1 for the cis-isomer, k′ = 3·16 × 10 −4 l mol −1 sec −1 and k″ = 0·216 × 10 −4 sec −1 for the rans-isomer. E a = 21·0 ± kcal/mol −1 and log PZ = 15·0 ± 0·7 for the cis-isomer, E a = 23·0 ± 0·7 kcal/mol −1 and log PZ = 16·6 ± 0·5 for the trans-isomer. The mechanism for both reactions is suggested to be similar to that of the aquation of cis-diaquabis(malonato)chromate(III) anion.