A double metal-cyanide catalyst based on Zn-3[Co(CN)(6)](2) was prepared. This catalyst is very effective for the ring-opening polymerization of propylene oxide, Polyether polyols of moderate molecular weight having low unsaturation (<0.015 meq/g) can be prepared under mild conditions. The molecular weight of polymer is entirely controlled by a reacted monomer-to-initiator ratio. The polymers prepared with stepwise addition of monomer exhibit a narrower molecular weight distribution as compared with those prepared with one-step addition of monomer. Various compounds containing active hydrogen, except basic compounds and low-carbon carboxylic acid, may be used as initiators. The reaction rate increases with increasing catalyst amount and decreases with rising initiator concentration. Polymerization involves a rapid exchange reaction between the active species and the dormant species. It was also proven that, to a certain extent, the chain termination of this catalytic system is reversible or temporary. C-13 NMR analysis showed that the polymer has a random distribution of the configurational sequences and head-to-tail regiosequence. It is assumed that the polymerization proceeds via a cationic coordination mechanism.