AbstractPolymerization of ethylene oxide (EO)‐catalyzed by a binary system consisting of triphenyl‐antimony dibromide (TPAB) and triphenylphosphine (TPP) was carried out. TPAB itself could induce the polymerization, but the addition of TPP to TPAB greatly enhances the catalytic activity. A characteristic inflection point, which appeared clearly in the time‐polymerization curves, divided the polymerization into two successive steps. The preceding process is a cationic step in which an intermediate antimony dialkoxide is produced and the following a coordinated polymerization. The mean consumption of EO per catalyst at the inflection point was calculated to be 45 and was independent of the mole ratio of [TPP]/[TPAB]. It is assumed that TPP coordinates to the central antimony atom to produce an active complex salt in the second step. The effect ofp‐substitution of TPAB in the polymerization was also investigated; the Hammett parameterspfor the first and the second polymerization step were found to be +1,7 and +1,0, respe