The kinetics of permanganate oxidation of β-phenylalanine in aqueous perchlorate media at a constant ionic strength of 3.0 mol dm–3 has been studied spectrophotometrically. A first-order reaction in [MnO4–] and a fractional order with respect to β-phenylalanine were observed. The results obtained at various hydrogen-ion concentrations indicate that the protonated species of both the amino acid and permanganate play a role in the kinetics. Kinetic evidence for the formation of an intermediate complex between the amino acid and the oxidant is presented. The activation parameters were calculated and found to be ΔS‡=–160.14 ± 8.23 J K–1 mol–1, ΔH‡= 34.04 ± 1.02 kJ mol–1, and ΔG‡= 81.77 ± 3.47 kJ mol–1, respectively. A mechanism consistent with the kinetic results is discussed.