This article describes thermal risk assessment of vegetable oil epoxidation by peroxycarboxylic acid. It is a liquid-liquid system where several exothermic reactions occur. Acetic acid was used as the carboxylic acid, and oleic acid was chosen as a model molecule because it is a common fatty acid in the triglyceride molecule. Differential scanning calorimetry (DSC) and accelerating rate calorimetry (ARC) were used to determine safety criteria such as the final temperature (T (Final)), T (D24,) and time to maximum rate under adiabatic condition (TMRad). We found that the calculation of TMRad based on DSC data could be incorrect when assuming a zero-order kinetic reaction. By using a process temperature of 70 A degrees C, the extrapolated final temperature was found to be 544 A degrees C from DSC experiments, T (D24) was estimated to 20 A degrees C based on ARC experiment, and TMRad was calculated to 164 min from ARC experiments. These criteria indicate the process can lead to a thermal runaway. Therefore, we recommend that vegetable oil epoxidation by peroxycarboxylic acid should not be performed in batch reactor, but in semi-batch mode.