The effect of chloride ions (10(-7)-10(0) M) on the potential oscillation generated by the oxidation of formic acid is investigated with polycrystalline platinum at 315 K in a sulfuric acid solution containing 1 M formic acid. It is found that at very low concentrations of chloride ions (less than or equal to 10(-7) M), at a current near the maximum one for the appearance of oscillation, the potential shows small oscillations at and near the peak potential of a relaxational oscillation. In the chloride ion concentration range of 5 x 10(-7)-10(-5) M, a chaotic oscillation appears in the portion of the small oscillations. When the chloride ion concentration is 5 x 10(-5) M or above, the potential shows a very stable periodic oscillation with an amplitude larger than 0.5 V and without small oscillations. In particular, at a concentration higher than or equal to 10(-2) M, the increasing potential velocity in the oscillation becomes faster than the decreasing one, although the reverse is true at a lower concentration. A bifurcation diagram is sketched with the applied current as the abscissa and the chloride ion concentration as the ordinate. The reasons for the appearance of the small oscillations and the large-amplitude oscillations are discussed on the basis of the voltammograms measured systematically. (C) 2008 Elsevier B.V. All rights reserved.