We have determined the effect of proton concentration on the appearance of chaos and windows of periodic oscillations in the galvanostatic oxidation of formaldehyde on platinum at 43 degrees C. When proton concentration increases from 0.1 to 5 mol dm(-3), the highest currents at which periodic oscillations and chaos appear decrease, and the oscillation amplitudes increase, while the oscillation periods change in a complex way. When perchloric acid is used instead of sulfuric acid, the highest currents at which oscillations appear increase by about 50%. These results (except those concerning periods) have been explained by using voltammograms and considering the reaction mechanism. We have found new types of alternating periodic and chaotic sequences, which connect the known sequences. We have used these findings to plot bifurcation diagrams.