The nature of ring-opening polymerization of oxetane derivatives initiated by triflic acid and triflic anhydride was investigated via F-19-NMR spectroscopy. Two types of propagating species, which were directly observed from the F-19-NMR spectra, were found to be oxonium macroions and triflic macroesters. These two propagating species established an equilibrium relationship during the polymerization process. The forming of oxonium macroion active species was highly dependent upon the substituents of the oxetane ring, and were enhanced when a more polar solvent was employed. The initiator effect, which occurred primarily in the initiating stage of the polymerization, is also discussed.