The reduction of permanganate by oxalate in the presence of manganese(II) ion in acidic media is described. All reactions were run at 525 nm and constant ionic strength 1.0 M. The reaction was found to obey the rate expression — d[MnO 4 -] d t = k [Mn 2+] [C 2O 4 2-] 2 [MnO 4 -] [H +] -2 = k' [MnC 2O 4] [MnO 4 -]. The values of k and k' were shown to be 5.4 × 10 4 M -1 s -1 and 8.2 × 10 4 M -1 s -1, respectively. Reaction rate methods for the determination of manganese(II) and oxalic acid are reported. The rate of disappearance of permanganate was monitored automatically and related directly to manganese-(II) and oxalic acid concentrations. Manganese(II) in the ranges 1–10 × 10 -4 M and 1–10 × 10 -3 M and oxalic acid in the range 0–20 μg ml -1 can be determined very rapidly with a precision of 1–2%.