The appearance of chaos and windows of periodic oscillations in the galvanostatic oxidation of formaldehyde on platinum at 43 degrees C is studied using various applied currents and formaldehyde concentrations. Potential oscillation patterns change with time in such a way that, after a period-doubling cascade, a sequence of periodic oscillations interposed between chaotic states appears. The sequence of periodic oscillations occurs in two ways, that is, in descending and ascending orders of their periods. The descending-order sequence is observed at comparatively low formaldehyde concentrations (0.003-10 mol dm(-3)), and the ascending one is observed at comparatively high concentrations (0.3-18 mol dm(-3)). At formaldehyde concentrations between 0.3 and 10 mol dm(-3), the ascending-order sequence always appears at higher currents than the descending one. We plot bifurcation diagrams for these two order sequences where one axis is time, which is related to surface states. The next-minimum return map for chaotic oscillations shows a tendency to become more nonuniform with time, which tendency is confirmed by calculating the variance in local Lyapunov exponents. Current oscillation is related to the potential oscillation. Negative resistance hidden in an ordinary cyclic voltammogram is found when the potential sequence before oscillation occurs (an induction period) is simulated. The species responsible for the negative resistance is thought to be adsorbed water adjacent to adsorbed carbon monoxide.