The oxidation of organic compounds with nitric acid has the potential to undergo a runaway hazardous reaction and explosion after the thermal-neutral induction period. The purpose of this study is to obtain a better understanding of the oxidation reaction mechanism to clarify the specific conditions under which the exothermic reaction occurs. In this study, the oxidation reactions of formic acid with nitric acid were investigated to represent a typical mixture of organic compound and nitric acid. From the results of thermal analysis, the overall reaction rate in a 15% nitric acid and 10% formic acid aqueous solution was formulated using an autocatalytic reaction model. The apparent activation energy was 75.2 kJ mol(-1), and the pre-exponential factor was 7.87 x 10(8) s(-1). The reaction rate increased with either nitric or formic acid concentrations, emphasizing that it is important to treat the mixture at lower concentrations for thermal safety. As a result of the thermal and in situ Raman spectroscopic analyses, it was found that the concentration of nitrous acid increased with the progress of the reaction as nitric acid and formic acid changed into NO2, N2O4, HCO(O)NO, HCO(O)NO2, and so on. The reaction mechanism started from nitrous acid and the oxidation of formic acid with nitric acid is an autocatalytic reaction involving this nitrous acid-initiated cycle.