A number of new poly(organophosphazenes) have been synthesized which bear 2-butenoxy or (4-(allyloxy)phenyl)phenoxy side groups. Cosubstituent groups included trifluoroethoxy, phenoxy, or (benzyloxy)phenoxy groups. Species with (4-(allyloxy)phenyl)phenoxy units underwent Si-H coupling with linear silanes or siloxanes to extend the side groups and form hybrid phosphazene/organosilicon polymers. A number of these polymers are rubbery elastomers which are readily cross-linked by heat or light. Seven of the mixed-substituent, cross-linked polymers were incorporated into interpenetrating polymer networks (IPN’s) with polystyrene, poly(methyl methacrylate), polyacrylonitrile, poly(acrylic acid), and poly(dimethylsiloxane). The phase compatibility characteristics of the IPN’s were assessed by DSC, TEM, FT-IR spectroscopy, and H-1 and P-31 NMR spectroscopy data.