The number of main chemical reaction intermediates in electrochemical oxidation of formaldehyde on a platinum electrode is determined to be three by analyzing the chaotic patterns of potential oscillations with a constant current. In a chaotic situation, the number of independent variables, which is the number of reaction intermediates appearing near the rate-determining step, is between the first or second integer greater than the correlation dimension and the minimal necessary embedding dimension. For the reaction studied, the upper and lower limits for the number of independent variables are both found to be three from the Grassberger Procaccia plot. This agrees with the result derived from chemical measurements and is thought to be a good base for mathematical modeling of the reaction system.