Four new co-substituted linear phosphazene polymers have been synthesized with controlled ratios of oligoethyleneoxy (MEEP-type) and trifluoroethoxy side groups. The polymers were synthesized as models for gel electrolyte systems with more dimensional stability than can be achieved by MEEP-type electrolytes alone. The conductivity is significantly influenced by the ratios of the two side groups and by the concentration of small-molecule organic additives. Insights are suggested into the mechanism of ionic conduction within polyphosphazene gels.