1. According to the data presented, all of the compounds tested which exhibit marked antioxidant activity have an Ec value between 0.65 volt and 0.90 volt. Many substances exist which have an Ec value within the optimum range, but do not have much antioxidant activity. The condition that the Ec value for a good antioxidant falls between 0.65 volt and 0.90 volt seems to be necessary but not sufficient. 2. A value of Ec within an optimum range seems to be only one of several possible physico-chemical phenomena which govern the rubber antioxidant activity of a compound. 3. The determination of the Ec value of a compound should not be used as the sole criterion of that compound's antioxidant activity.