The results of emulsion polymerizations in which twenty‐eight different organic hydroperoxides were used as oxidizing agents in a standard redox recipe at 5°C. have been presented. Each hydroperoxide was tested at several different concentration levels. The hydroperoxides of chlorodiisopropylbenzene, di‐t‐butylisopropylbenzene, cyclohexylbenzene, 1,2,3,4,4a,9,10,10a‐octahydrophenanthrene, and t‐butylisopropylbenzene were found to be the most effective of the compounds tested. Oxidation mixtures, concentrates of oxidation mixtures, and purified samples of hydroperoxides served as sources of hydroperoxide for the polymerization experiments reported. Similar results were obtained with oxidation mixtures and concentrates, but purification of the hydroperoxide usually resulted in improved polymerization rates. The amount of hydroperoxide which gives the optimum results in a given polymerization recipe varies for structurally different hydroperoxides and appears to be related to the molecular weight.