We report on the effect of addition of low molecular weight (M-w, = 400) PEG on protonic conduction of the solid polymer electrolyte (PEG)(100)NH4ClO4. We study the changes in the temperature dependence of conductivity as a function of the amount of the added plasticizer. NMR lineshape studies indicate a reduction in the relative fraction of crystalline regions on addition of the plasticizer. We show that the plasticizer reduces both the T-g, and crystallinity of the samples, thus increasing the conductivity.