In this work, the kinetics of soybean oil epoxidation with hydrogen peroxide in the presence of formic acid as an oxygen carrier and phosphoric acid as a catalyst has been studied. A biphasic kinetic model developed in a previous work has been improved considering the evolution of four different reactions with time: (1) the oxidation of formic acid to performic acid, occurring in aqueous phase; (2) the epoxidation reaction, occurring in oil phase; (3) the ring-opening reaction, mainly occurring at the liquid-liquid interface; and (4) the performic acid decomposition, occurring in aqueous phase. The kinetics of reactions 1, 3, and 4 have been studied in an independent way, and the best kinetic parameters found and published elsewhere have been used for interpreting the experimental results obtained in performing reaction 2. A kinetic law for this reaction has been proposed, and the related best-fitting parameters have been determined.