Thermal polymerization of cyclotriphosphazenes fully substituted with 2-, 3-, and 4-pyridinoxides have been attempted both in the solid state and in solution. Among these ring-strain-free trimeric isomers, hexakis(3-pyridinoxy)cyclotriphosphazene (1) did not undergo polymerization either in the solid state or in solution, but 4-pyridinoxy (2) and 2-pyridinoxy analogues (3) have been found to undergo polymerization at 150 and 200 degrees C, respectively, in the absence of any catalyst. In particular, the thermal solution polymerization of 2 in 1,2,4-trichlorobenzene at 200 degrees C resulted in a linear polymer in 80% yield which was the same as the product obtained by a nucleophilic substitution of poly(dichlorophosphazene) with 4-pyridinoxide. DSC analysis of the trimers has shown that 2 and 3 have exothermic peaks at 179.4 and 224.1 degrees C, respectively, whereas 1 does not have an exothermic peak, and it has been found that the polymerizability and temperature range for polymerization of the cyclic trimers are closely related to their thermal properties: trimers having an exothermic peak are amenable to the thermal polymerization reaction, and the temperature range for polymerization can be predicted therefrom. Along with such a finding, the results of the thermal conductivity measurements of trimers 1, 2, and 3 conclusively support the cationic ring-opening mechanism. The difference in polymerizability of these trimeric isomers may be explicable in terms of an electronic difference of the substituent isomers: 4- and 2-pyridinoxide anions are more resonance stabilized than the 3-pyridinoxide anion, which probably makes a difference in the degree of ionization of the substituents from the trimeric phosphazene ring at the initial step of the ring-opening polymerization.