Solid polymer electrolytes based on amorphous polyether-urethane networks combined with lithium or sodium salts and a low molecular weight cosolvent (plasticizer) have been investigated in our laboratories for several years. Conductivity enhancements of up to two orders of magnitude can be obtained whilst still retaining solid elastomeric properties. In order to understand the effects of the plasticizers and their mechanism of conductivity enhancement, multinuclear NMR has been employed to investigate ionic structure in polymer electrolyte systems containing NaCF3SO3, LiCF3SO3 and LiClO3 salts. With increasing dimethyl formamide (DMF) and propylene carbonate (PC) concentration the increasing cation chemical shift with fixed salt concentration indicates a decreasing anion-cation association consistent with an increased number of charge carriers. C-13 chemical shift data for the same systems suggests that whilst;DMF also decreases cation-polymer interactions, PC does the opposite, presumably by shielding cation-anion interactions. Temperature dependent Li-7 spin-lattice relaxation times indicate the expected increase in ionic mobility upon plasticization with a shift of the T-1 minimum to lower temperatures. The magnitude of T-1 at the minimum increases upon addition of DMF whereas there is a slight decrease when PC is added. This also supports the suggestion that the DMF preferentially solvates the cation whereas the action of PC is limited to coulomb screening, hence freeing the anion.