We describe the use of a decationated Y zeolite as a promising catalyst for the epoxidation of soybean oil with hydrogen peroxide and formic acid. The pores of this zeolite are too narrow for the oil molecules that remain outside the zeolite framework (shape selectivity), while, all the molecules of the aqueous mixture can enter and react inside the pores to form in situ the reactant performic acid, the formation of which is catalyzed by an acid environment. Because of the absence of mineral acid in the aqueous solution, outside the pores, the undesired side reactions of ring opening are limited. The most opportune operative conditions and zeolite pretreatments for improving activity and selectivity will be described.