The oxygenation constants (Ko2) of cobalt(Ⅱ) hydroxamates (CoL12-CoL32) with benzo-15-crown-5 (B15C5) pendants were measured over the range of -5 to +20 ℃, and the values of thermodynamic parameters (△H0 and △S0) were calculated based on these (Ko2) values. Meanwhile, these crowned complexes were employed to the oxidation for p-xylene to p-toluic acid with air at 110△ under normal atmospheric pressure. The effects of B15C5 pendant and the length of chain bonded to B15C5 in these complexes on the O2-binding capabilities and oxidafon for p-xylene were investigated with the comparison of crown-free analogues CoL24.