AbstractA study of a series of metal compounds as promoters of acyl peroxide and hydroperoxide dissociation in an unsaturated polyester‐styrene resin solution has shown vanadie and vanadyl chelates to be the most active. All of the results were compared to a standard cobalt‐methyl ethyl ketone peroxide promoter‐catalyst system. It was found that the reactivity of 2 mg. of vanadium or vanadyl ion as vanadic or vanadyl acetyl acetonate with respect to 1% cumene hydroperoxide was greater than 24 mg. of cobalt as cobalt naphthenate with respect to 1% methyl ethyl ketone peroxide. This is significant, in that considerably less metallic promoter can be used at ambient temperatures for the fabrication of finished articles with considerably less associated color. Of the vanadium compounds studied, vanadic and vanadyl chelates were about equally reactive. Vanadyl acetyl acetonate was considered the promoter of choice, since vanadic acetyl acetonate solutions appeared to be air‐oxidized readily. Study has shown that benzene solutions of vanadic acetyl acetonate lost some activity after a week, whereas chloroform solutions of vanadyl acetyl acetonate remained stable for a period of six to nine months. The utility of vanadium promoters for ambient temperature redox polymerization would be enhanced if they could be incorporated into unsaturated polyester resins at the time of preparation. However, results obtained indicated that both vanadic and vanadyl chelates limit the stability of such prepromoted resins, normally leading to gelation in about 17 days. A further indication of the utility of vanadium chelates as redox promoters was the observation that these materials are reactive towards diacyl peroxides as well as hydroperoxides. Vanadium chelates were the only materials studied which showed this wide spectrum of rea