The stoichiometry of the Ce(IV) oxidation of malonic acid (MA) under aerobic and anaerobic conditions was investigated. The Ce(IV)/MA consumption ratio is found to vary from about 3.5 to 7, depending nonlinearly on the initial concentration of malonic acid as well as dissolved oxygen. The observed data can be quantitatively explained by a model that employs the characteristic structure of branching pathways in the overall mechanism and does not require the exact knowledge of the rate constants. It is concluded that a systematic study of consumption ratios provides an important aid for elucidating mechanisms of complex reactions.