The oxidation of arsenic (III) by chromium (VI) has been studied in pyridine - pyridinium buffers with and without added oxalate ion. The rate law at pH 4.6 for the reactions without oxalate is best described as - d[Cr(VI)]/dt = (0.535 [As(III)]² [Cr(VI)])/(1+187 [As(III)]) This is in accord with the general mechanism for chromium (VI) oxidation proposed by Beattie and Haight. The behavior in pyridine - pyridinium buffers is different from that observed in acetic acid acetate buffers of the same pH. In the presence of oxalate, the observed rate law is - d[Cr(VI)]/dt = (310 [As(III)] [C₂O₄²⁻] [Cr(VI)])/(1+31.0 [As(III)]) In each case the reaction stoichiometry was found to be 3As(III) + 2Cr(VI) = 3As(V) + 2Cr(III). The role of oxalate was found to be catalytic although it was incorporated in the final Cr(III) product. Complex kinetic behavior was observed as a function of pH, [AsIII], and buffer composition. / M.S.