Poly(organophosphazenes) containing 4-(allyloxy)phenoxy and 4-[4’-(allyloxy)phenyl]phenoxy side groups were synthesized and modified by hydrosilylation reactions. A small-molecule model compound, pentaphenoxy[4-(allyloxy)phenoxy]cyclotriphosphazene, underwent platinum-catalyzed hydrosilylation reactions with heptamethyltrisiloxane and dimethylethoxysilane. The product of the second reaction, containing one dimethylethoxysilane group, underwent hydrolysis and self-condensation reactions in the presence of acid. Transposition of this chemistry to the high-polymer level was then accomplished. Thus, hydrosilylation of phosphazene high polymers with allylaryloxy side groups yielded poly(organophosphazenes) with dimethylsiloxane side group graft units. High siloxane loadings were obtained without evidence of Si-O or Si-C bond cleavage. A series of polymers with 4-100% grafting of dimethylsiloxane segments (corresponding to 6.5-56.2% dimethylsiloxane by weight) were synthesized and characterized by DSC, TGA, and GPC. Two polymers were further characterized by DMA. Glass transition temperatures of the siloxane-containing polymers ranged from -79 to +20 degrees C, according to the amount of siloxane grafting and the nature of the other groups attached directly to the phosphazene polymer backbone. The siloxane-containing polymers also showed a relaxation at approximately -120 degrees C. This combined with SAXS data suggested that the dimethylsiloxane side group may exist in microphase-separated domains. Cross-linking of polymers that contained unsaturated groups was achieved by ultraviolet irradiation and gamma-irradiation. Poly(organophosphazenes) bearing 20 and 46 mol % dimethylethoxysilane groups cross-linked readily on exposure to atmospheric moisture.