A simple phenolic lignin model compound (syringyl alcohol, SA) was subjected to the reaction with a one-electron oxidizing reagent, Mn3+ (manganese(III) acetate), in a weakly acidic aqueous medium or in acetic acid medium under various oxygen pressure (0 to 1.0 MPa). Yields of the reaction products, especially that of methanol, were used to evaluate the progress of oxidation reaction and the difference of oxidation mechanism due to the presence of molecular oxygen. The difference in the oxygen pressure did not result in any detectable difference in reaction products, suggesting that molecular oxygen did not participate in the oxidation process which were initiated by one-electron abstraction from phenolic hydroxyl group. Contrary to this, a clear effect of oxygen pressure on the methanol yield was observed when methanol was determined after the reaction mixture being subjected to the alkali post-treatment. This is attributed to the generation of some active species from the reaction of Mn2+ with molecular oxygen and to the simple oxygen-alkali oxidation.