The oxidative decarboxylation of 3,4-methylenedioxymandelic acid (DMA) to piperonal with dilute nitric acid was investigated. This reaction was initiated by NaNO2 and catalyzed by H2SO4, but the autocatalytic behavior, which was noted in most nitric acid oxidation reactions, was not observed. Detailed kinetic study showed that the respective reaction orders of NaNO2, H2SO4, HNO3, and DMA were 0.5, 0.5, 1.2, and 1.8, and the activation energy was 59.65 kJ/mol. The gained reaction rate equation provided a good model for the reaction under other temperatures and even the condition of a slightly higher concentration of nitric acid. Additional experimental data suggested that HNO2 was the reactive species. On the basis of the results, a possible reaction mechanism was proposed.