Electronic mechanisms of reductive activation of O2 by oxidases coenzymes (K) are considered. Activation is induced by triplet-singlet (T-S)-transitions at the charge-transfer stage and K+...O2- radical pair (RP) formation. For flavoproteins the spin reversion is induced by spin-orbit coupling (SOC) in superoxide ion, for metal-oxidases--by exchange interaction with paramagnetic metal ion. Another type of spin activation is connected with the singlet peroxide ion O(2)2- formation in cytochromes (structures of the type Fe(3+)-O- -O-Cu2+).