The terpolynlerization of epichlorohyd rin，ethylene oxlde and propylene oxide with triisobutylaluminium．phosphoric acid-nitrogen．containing electron donor as cata-lyst has been studied．It is shown that as the third eomponent of the catalyst di-methylaniline is the best，tetramethylthiuram disulfide or triethylamine the next．and benzothiazole the worst．While pyridine is used as the third component of the catalyst，the inherent viscosity of the terpolymer obtained is low The catalytic acti-vity is higher when water is presefit in equal moles to phosphoric acid．It is indicated that at the beginning of the terpolymerization ethylene oxide and propylene oxide polymerize more quickly and epichlorohydrin polymerizes slvwly．With increase of propylene oxide in the monomer mixture，the inherent viscosity of the terpolymer decreases and the glass transition temperature passes through a minimium value．