The results are presented on the synthesis of new modifiers and on functioning of these modifiers in unsaturated polyester (UP) resins. The modifiers were obtained by reacting ethylene oxide or propylene oxide with phenyl-1,3-diamine. The effect of the modifiers and cobalt accelerator on the reactivity of resins was studied. When used in the amount of up to 1.50 wt %, the amines substantially (several times) reduced the gelation time of modified UPs. The reactivity of resins, however, improved in expense of their stability. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012