The cyclic carbophosphazone N3P2CCl6 undergoes thermal ring-opening polymerization at 120-degrees-C to yield poly(pentacblorocarbophosphazene), [N3P2CCl5]n, as a hyclrolytically sensitive elastomer. Treatment of this polymer in solution with primary and secondary alkylamines yielded poly(aminocarbophosphazenes) [N3P2C(NR1R2)5]n where R1 = H, R2 = (n)Pr or (i)Pr, and R1 = R2 - Me or Et. These polymers are also sensitive to moisture. An anilino/dimethylamino mixed-substituent polymer, [NsP2C(NHPh)x-(NMe2)y(Cl)z]n (x = 1.50, y = 3.34, z = 0.16), and a 2,6-phenylphenoxy/dimethylamino mixed-substituent polymer, [N3P2C(2,6-OPh(Ph)2)x(NMe2)y]n (x = 1, y = 4), were synthesized via the same technique. A mixed-substituent polymer with regiospecific backbone carbon substitution, [(NP(OCH2CF3)2)2NC(N(Ph)2)]n, was also synthesized. These mixed-substituent polymers are stable to water. The macromolecules were characterized by P-31, H-1, and C-13 NMR spectroscopy, IR spectroscopy, elemental analysis, differential scanning calorimetry, thermogravimetric analysis, and gel permeation chromatography. Poly[(dimethylamino)carbophosphazene] undergoes a depolymerization reaction in solution and in the solid state to give small-molecule cyclic analogues, including N3P2C(NMe2)5. The depolymerization reaction was investigated by P-31 NMR spectroscopy and thermogravimetric analysis in conjunction with mass spectrometry. Analogous small-molecule model reactions were undertaken in which the cyclocarbophosphazene N3P2CCl6 was allowed to react with primary and secondary alkylamines to yield the cyclic species N8P2C(NR1R2)5. The mixed-substituent cyclic species {[NP(OCH2CFs)2]s[NC(N(Ph)2)]} and {[NP(NMe2)2][NP(2,6-OPh(Ph)2)(NMe2)][NC(NMe2)]} were also synthesized.