The polyphosphazene MEEP, [NP(OCH2CH2OCH2CH2OCH3)(2)](n), was first reported 12 years ago as a significant advance in the design of solid polymer electrolytes (P. M. Blonsky,D. F. Shriver, P. E. Austin and H. R Allcock, J. Am. Chem.;Soc. 106, 6854 (1984)) [1]. Since that time, more than 30 new polymers have been synthesized in our laboratory, which build on the basic structure and theory developed for MEEP. The new polymers possess a phosphazene backbone, which is the basis of the molecular flexibility, with a variety of different alkyl ether and alkoxy side groups. The ionic conductivity of these polymers in the presence of lithium triflate has been studied as a function of the different polymer side groups. Several systems that improve on the properties of MEEP have been identified. The behaviour of selected polymers in operating rechargeable batteries has also been investigated.