The effect of chloride ions (10(-7) to 10(-1) M) on the potential oscillation generated by the oxidation of formaldehyde is investigated with polycrystalline platinum at 315 K in a sulfuric acid solution containing 0.1 M formaldehyde. When the chloride ion concentration is 10(-5) M or lower, between the two current regions for the appearance of relaxational oscillations, a large-amplitude trapezoidal oscillation appears, which is alternately accompanied by a relaxational one at a lower current. At a still lower current, the relaxational oscillation changes to a sinusoidal one, which then produces a well-known chaos. When the chloride ion concentration is between 10(-4) and 3 x 10(-4) M, the relaxational oscillations disappear and the alternating oscillation shows chaos, the properties of which are different from those of the chaos already stated. When the chloride ion concentration is 10(-3) M or greater, a simple large-amplitude trapezoidal oscillation prevails without alternation with the relaxational one, along with much less chaos. The surface adsorption state during the various observed oscillations is discussed on the basis of voltammograms measured at various chloride ion concentrations. (c) 2008 Elsevier Ltd. All rights reserved.