The synthesis of new hybrid polyphosphazene-siloxane polymers is described. These polymers contain a polyphosphazene backbone and [(N-methylamino)propyl]siloxane side groups, -N(Me)(CH2)(3)SiMe(2)OSiMe(3) or -N(Me)(CH2)(3)SiMe(2)OSiMe(2)OSiMe(3), in ratios of 1:1 and 1:3 with trifluoroethoxy, 2-(2-methoxyethoxy)ethoxy, or p-methylphenoxy cosubstituents. The synthetic method utilized allows the polymer properties to be tuned by variations in the cosubstituent ratios. The polymers were characterized by P-31, C-13, and H-1 NMR, differential scanning calorimetry (DSC), gel permeation chromatography (GPC), and elemental analysis. The percent loading of siloxane-containing side groups and the nature of the cosubstituents were correlated to the glass transition temperatures. The glass transition temperatures ranged from -83 to -22 degrees C. The longer, trisiloxane-containing units generated lower glass transition temperatures than did the analogous polymers with disiloxane-containing side groups. Several polymers were further characterized by dynamic mechanical analysis (DMA). In addition to glass transition temperatures, other transitions attributed to the siloxane side groups were detected at approximately -124 degrees C.