The stereochemistry of alternating copolymerization of carbon dioxide with S‐(–)‐3‐phenyl‐1,2‐epoxypropane (5) using a diethylzinc/water system as catalyst was investigated. The optically active copolymer, poly[(oxycarbonyloxy)‐1‐benzylethylene] (6), was hydrolyzed and the epoxide unit was transformed into the form of 1,2‐bis(trimethylsilyloxy)‐3‐phenylpropane (8). From the determination of optical activity of this ether, it is concluded that the ring opening of 5 in the copolymerization takes place predominantly at the methylene‐oxygen linkage. The substituent effect on the ring opening mode of oxiranes is also discussed.