Abstract A kinetic study of the chromium(II) reduction of (O-bonded amino acid)pentaamminecobalt(III) complexes has been carried out in acid solutions. The second-order rate constants in M−1 s−1 (1 M=1 mol dm−3) at 25 °C and I=1.0 M (NaClO4) for α-alanine, phenylalanine, α-aminoisobutyric acid, β-alanine, β-aminobutyric acid, γ-aminobutyric acid, and ε-aminohexanoic acid complexes are 1.8×10−2, 1.5×10−2, 2.4×10−3, 7.5×10−2, 5.0×10−2, 1.22×10−1, and 1.49×10−1 respectively. The values of ΔH\eweq lie in the range of 6.4–7.7 kcal mol−1 (1 cal=4.184 J), and those of ΔS\eweq, in the range of −42–−37 cal K−1 mol−1, except for the α-aminoisobutyric acid complex, the ΔH\eweq and ΔS\eweq values of which are 11.4 kcal mol−1 and −32 cal K−1 mol−1 respectively. The role of the amino acid ligands in the inner-sphere electron-transfer reactions is discussed.