The influence of the donor complexing additive o-phenanthroline (Phen) on the antiradical activity of some sulfur-containing Cu, Ni, and Zn chelates was studied in the model reaction of initiated oxidation of ethylbenzene (333 K). The inhibition rate constants fk(7) for the chelates and effective inhibition rate constants fk(7)* for 1 : 1 chelate-Phen mixtures were determined from initial oxidation rates. The activating effect of Phen is not observed for chelates with the strongest donating sites (alpha = fk(7)*/fk(7) = 1) and regularly increases up to (x = 4 as the donating ability of the site decreases. The inhibition rate constant was found to nonmonotonically depend on the Phen : chelate concentration ratio in the mixture, the maximum value of fk(7)* being observed at the ratio of 1 : 1 or 2 : 1 in the case of a chelation donor site with a lower or a higher donating ability, respectively. Based on spectrophotometric data, the negative effect of Phen on the overall inhibiting action of Ni diethyldithiocarbarnate was explained.