The polymerization of dioxolane by triethyloxonium hexafluorophosphate in methylene chloride has been studied with a view to determine the nature of the active center. NMR studies of solutions of the initiator with low ratios of monomer led to little reaction of the initiator over long times. Analysis of normal reaction mixtures showed that only a small amount of the initiator was consumed during the reaction. The polymer was studied by GPC, UV spectroscopy, and NMR. The polymer appears to consist largely of high molecular weight material with Mw/Mnless than 2, and also low molecular weight material perhaps formed by a different mechanism. The high molecular weight material appears not to have been formed by a simple linear trialkyloxonium ion, from end-group studies, and it is suggested the active center is a secondary oxonium ion on a large cyclic polymer.