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Effects of imidazoles and substituted pyridines and pure s-donors on the oxidation of cyclooctene with NaIO4 by 5,10,15,20-tetraphenylporphyrinato-manganese(III) acetate, Mn(TPP)OAc, and 5,10,15,20-Tetramesitylporphyrinato-manganese(III) acetate, Mn(TMP)OAc, catalysts in CH2Cl2-H2O solvent are studied. Coordination of the nitrogen donors to the Mn-catalysts generally caused a reduction in the epoxidation rates, and an increase in their stabilities toward oxidative degradation. Steric properties of the donors rather than their basicity seems to be more dominating in these oxygenation reaction. The bulky Mn(TMP)OAc showed a greater steric sensitivity than Mn(TPP)OAc.