Advancement catalysts which are a combination of a phosphonium halide or an alkylene phosphorane and a nitrogen heterocycle selected from the group consisting of substituted or unsubstituted imidazoles, benzimidazoles, imidazolines, dihydropyrimidines, tetrahydropyrimidines, dihydroquinazolines, their salts and mixtures thereof, confer high reactivity and selectivity on epoxy resin advancement reactions. The advanced epoxy resins are useful in the preparation of coatings of high quality.