Using EPR spectroscopy, high valence Mn(salen) complexes were monitored in Mn I I I (salen)/isobutyraldehyde/O 2 and Mn I I I (salen)/pivalaldehyde/O 2 catalytic systems in the absence and in the presence of N-methylimidazole (N-Me-Imd), where Mn I I I (salen) (1) is (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino-man ganese(III) chloride. Interaction of 1 with aliphatic aldehydes in the presence of O 2 was found to give a mixture of oxo manganese(IV) species L 1 (salen)Mn I V =O (2) with various axial ligands L 1 (L 1 =Cl - , RCOO - ). They display EPR signals having close g values of ca. 5.2 with resolved hyperfine structures from one manganese (A=75G, 5 5 Mn, I=5/2). In the presence of N-Me-Imd, new manganese(IV) complexes were detected. The first one was shown to be an antiferromagnetically coupled mixed-valence [(salen)Mn I V (μ 2 -O) 2 Mn I I I (salen)] - dimer having 16-line EPR signal at g=2. The second one is probably (salen)Mn I V (OOCOR)N-Me-Imd acylperoxo complex (2) (g=4.2 (R=CH(CH 3 ) 2 ), or g=4.7 (R=C(CH 3 ) 3 ), unresolved manganese hyperfine structure). Possible roles of each complex in the enantioselective alkene epoxidations are discussed.