The thermal stability of solutions of methyl ethyl ketone peroxide (MEKPO) dissolved in dimethyl phthalate (DMP), in the presence of nitric acid (HNO3) and sodium nitrate (NaNO3) as contaminants, was studied. MEKPO is extensively employed in the chemical industries, but despite its large use several severe accidents have been recorded in eastern Asia during the past four decades. This study was conducted to elucidate its essentially hazardous characteristics. We used differential scanning calorimetry (DSC) to evaluate the root cause of the runaway reactions, and kinetics-based curve fitting to assess hazardous phenomena by utilizing curve fitting to optimize the kinetic parameters. All the results indicate that the mixture of MEKPO with HNO3 dramatically increases the degree of hazard in the first exothermic peak. In terms of NaNO3, it had no effect on the thermal stability of MEKPO.