AbstractCobalt promoters presently used in the polyester processing industry suffer from many shortcomings ranging from shelf instability to strong discoloration of the resins and relatively poor activity. This paper describes the various performance improvements obtainable with coordination complexes of cobalt which can be used as polymerization accelerators in presence of organic peroxide initiators. These complexes appear to present significant advantages over the conventional cobalt carboxylates. Catalytic activity up to ten times that of cobalt soap was recorded with some complexes in various resin systems, without deleterious effects on physical properties. As gel‐cure modifiers, the compounds evaluated seem to increase the long term shelf stability of pre‐promoted resins while reducing the discoloration and air inhibition during the curing process of polyester thin films.