A series of poly(fluoroalkoxyphosphazenes) containing a mixture of 2,2,3,3,4,4,5,5-octafluoropentoxy and 2,2,2-trifluoroethoxy substituents was synthesized. The series included polymers with 25-94% incorporation of the trifluoroethoxy substituent as well as single-substituent polymers with 100% octafluoropentoxy or trifluoroethoxy side groups. Polymers were analyzed by multinuclear NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry, and thermogravimetric analysis, and were subjected to limiting oxygen index and microtensile testing. It was found that the T-g and T-d values among the cosubstituted polymers varied little with changes in composition, but the mechanical properties varied over a wide range. (C) 2004 Wiley Periodicals, Inc.