Oxetane has been polymerized using both triethyloxonium hexafluorophosphate and triphenylmethyl hexafluorophosphate as initiators at −30 °C. The former gives a slow polymerization with a slow initiation process. The latter gives a rapid initiation and rapid initial polymerization which may slow down to rates similar to those found for the former initiator. The results are interpreted in terms of the formation of two types of tertiary oxonium ions. One type incorporates the oxetane molecule and is suggested to have a very rapid reaction with more oxetane. The second type does not involve the oxetane molecule but an ether linkage, such as in the polymer chain, and reacts only slowly with oxetane.