Reaction of the halogenated poly(thionylphosphazene) [NSOCl(NPCl2)(2)](n) (2a), which possesses a novel S(VI)-N-P backbone, with 2, 3, and 4 equiv of Na[OCH2CF3], followed by excess (BuNH2)-Bu-n, afforded mixed-substituent poly[alkoxy(amino)thionylphosphazenes] [NSO((NHBu)-Bu-n)(NPR2)(2)](n) (6) (a, R = 51% OCH2CF3, 49% (NHBu)-Bu-n; b, R = 76% OCH2CF3, 24% (NHBu)-Bu-n; c, R = 95% OCH2CF3, 5% (NHBu)-Bu-n) in which the polymers were regioselectively substituted with trifluoroethoxy substituents only at phosphorus and not at sulfur. The resulting moisture stable polymers were structurally characterized by P-31, H-1, C-13, and F-19 NMR spectroscopy. Molecular weights were in the range M-W = 3.3 x 10(4) to 1.4 x 10(5) and M-W = 1.9 x 10(4) to 8.2 x 10(4) according to GPC analysis in THF versus polystyrene standards. The thermal transition behavior was investigated by DSC, glass transition temperatures (T-g＇s) ranged from -30 to -14 degrees C, and no melt transitions were detected. Analysis of these new polymers by TGA showed that they were stable to weight loss up to ca. 250 degrees C under nitrogen at a heating rate of 10 degrees C/min.