The activated-monomer cationic ring-opening polymerization of ethylene oxide, initiated with ethylene glycol and using an acid-exchanged montmorillonite clay called Maghnite-H+ as an effective catalyst, was carried out to obtain the corresponding homopolymers with narrow polydispersity ratios. The molecular weights of the obtained polymers were controlled with the feed ratio of the monomer to the initiator. The effects of the amount of the catalyst and time on the polymerization yield and viscosity of the polymers were studied. The structure was confirmed with proton nuclear magnetic resonance and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 535-540, 2009