Phenolic antioxidants were studied extensively in the 1960s. During that time, the action mechanism might have seemed to be clarified completely. Based on our recent study, however, there remain some unclear points on the interpretation of substituent effects. In this work, o-methoxyphenols having m-substituent have been investigated in detail to make any m-substituent effect clear. It was found that the m-substituent of an m-substituted o-methoxyphenol may behave as if it were on the p-position. This fact is explained by the suggestion that the m-substituent affects its p-substituent, which then influences its o-position by resonance. That is, an m-substituent on the phenol can affect the antioxidant activity, if the phenol also has an o-substituent. Such a substituent effect is proposed as a ''secondary substituent effect''.