The autoxidation of a mixture of 1,2-disubstituted pyrazolidine-3,5-diones 1 and alkenes 2 in the presence of a catalytic amount of manganese(III) acetate dihydrate in air gave 4,4-bis(2-hydroperoxyalkyl)pyrazolidine-3,5-diones 3 in 75-96% yields. The structure of the bis(2-hydroperoxyethyl)pyrazolidine-3,5-dione 3 (R 1 =R 2 =Ph, R 3 =R 4 =4-FC 6 H 4 ) has been corroborated by an X-ray single crystallographic analysis. On the other hand, the manganese(III) acetate oxidation of a mixture of 1 (R 1 =R 2 =Ph) and 2 (R 3 =R 4 =Ph) at elevated temperature gave 4,4-bis(2,2-diphenylethenyl)-1,2-diphenylpyrazolidine-3,5-dione (4) in good yield.