Abstract The diphenylzinc‐butanone system was used as polymerization catalyst for some oxiranes in benzene solution at 60°C. This system is greatly influenced by the molar ratio of butanone to diphenylzinc, and the maximum catalytic activity for propylene oxide and ethylene oxide was found for a ratio of unity. GPC results strongly suggest the presence of more than one active species for the system. 13 C NMR analysis indicates that the resulting poly(propylene oxide) has a head‐to‐tail arrangement. For the polymerization of propylene oxide with butanone/diphenylzinc = 1, after an initial induction period, the reaction was first‐order with respect to monomer with k = 2·51 × 10 −5 s −1 . Ethylene oxide polymerizations using butanone/diphenylzinc = 1 and 5 were also first‐order with respect to monomer after an initial induction period with k = 7·80 × 10 −6 s −1 and k = 5·71 × 10 −6 s −1 , respectively. The diphenylzinc‐butanone system was not an effective catalyst for styrene oxide polymerization.