The P-phenyl ring in poly(methylphenylphosphazene), [Me(Ph)PN]n (1), was nitrated at the meta position using a mixture of nitric and sulfuric acids, producing the nitrophenyl derivatives [Me(Ph)PN]x[Me(3-NO2C6H4)PN]y (2). Between 10 and 60% of the phenyl rings were nitrated by varying the reaction times from 30 to 60 min. The nitro polymers were treated with NaOH to remove protons coordinated to the backbone nitrogen atoms and were then reduced by NaBH2S3 to the amino derivatives [Me(Ph)PN]x[Me(3-NH2C6H4)PN]y (3). Conversion to the N-amido-substituted polymers, [Me(Ph)PN]x{Me[3-N(H)C(=O)C2H5](C6H4)PN}y (4), was accomplished by reaction of 3 with propionyl chloride in the presence of Et3N. The new poly(phosphazenes) were characterized by elemental analysis, H-1, P-31, and C-13 NMR and infrared spectroscopy, size-exclusion chromatography, and differential scanning calorimetry. The unusual chemical stability of the phosphazene backbone to the rigorous chemical reactions and the effect of protonation are also discussed.