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The selective cationic isomerizations and ring-opening polymerizations of cyclic fivemembered dithiocarbonates (1a, 5-phenoxymethyl-1,3-oxathiolane-2-thione; 1b, 5-methyl-1,3-oxathiolane-2-thione; 1c, 5-ethyl-1,3-oxathiolane-2-thione; 1d, 5-chloromethyl-1,3-oxathiolane-2-thione) have been achieved by the selection of the catalysts, which have enabled the first facile preparations of 1,3-dithiolan-2-one derivatives (2a-d) and polydithiocarbonates (P1a-d). The mechanism of the selectivity was elucidated by the 1 H and 13 C NMR spectroscopic analyses for their reaction intermediates. The initiation steps of the reactions of 1a with TfOEt or TfOH were different, wherein the formation of the cyclic carbenium ion (6) via the ethylation of TfOEt on the thiocarbonyl sulfur and the oxonium cation ( 7) via the protonation of TfOH on the ether oxygen were observed, respectively. The cation 7 might not be electrophilic enough to be attacked by the monomer (1), which could explain the fact that the results in the reaction of 1 selectively gave the isomer with TfOH. The polymerization and isomerization should proceed predominantly via 6 and 7, respectively. The reactivities of 1a-d decreased in the order of 1b = 1c > 1a > 1d, which showed good agreement with the 1 H NMR chemical shifts of the methine protons of 6a-d.