Intermediates of chromium-salen catalyzed alkene epoxidations were studied in situ by EPR, H-1 and H-2 NMR, and UV-vis/NIR spectroscopy (where chromium-salens were (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino chromium(III) chloride (1) and racemic N,N'-bis(3,4,5,6-tetra-deuterosalicylidene)-1,2-cyclohexanediamino chromium(III) chloride (2)). High-valence chromium complexes, intermediates of epoxidation reactions, were detected and characterized by EPR and NMR. They are the reactive mononuclear oxochromium(V) intermediate (A) (CrO)-O-V(salen)L (where L = Cl- or a solvent molecule) and an inactive chromium-salen binuclear complex (B) which acts as a reservoir of the active species. The latter complex demonstrates an EPR signal characteristic of oxochromium(V)-salen species and H-1 NMR spectra typical for chromium(Ill)-salen complexes, and it is identified as mixed-valence binuclear L-1(salen)(CrOCrV)-O-III(salen)L-2 (L-1, L-2 = Cl- or solvent molecules), The intermediates (CrO)-O-V(salen)L and L-1(salen)(CrOCrV)-O-III(salen)L-2 exist in equilibrium, and their ratio can be affected by addition of donor ligands (DMSO, DMF, H2O, pyridine). Addition of donor additives increases the fraction of A over that of B. The same two complexes can be obtained with m-CPBA as oxidant. Reactivities of the (CrO)-O-V(salen)L complexes toward E-beta-methylstyrene were measured in DMF The L-1(salen)(CrOCrV)-O-III(salen)L-2 intermediate has been proposed to be a reservoir of the true reactive chromium(V) species. The chromium-salen catalysts demonstrate low turnover numbers (ca. 5), probably due to ligand degradation processes.