Dynamic mechanical thermal analysis (DMTA) has been used to study the effects of plasticizers on the mobility and homogeneity of a series of solid polymer electrolytes (SPEs). With reference to previously published results on similar systems containing LiCIO4 salts and tetraglyme as plasticizer, the effects of propylene carbonate (PC) on the glass transition temperature (T-g) of the SPE and on the distribution of relaxation times within the sample are discussed; at low plasticizer concentration PC has little effect on T-g as measured by DMTA in comparison with tetraglyme, and at higher plasticizer concentrations PC significantly broadens the mechanical relaxation behaviour indicating a greater degree of dynamical heterogeneity within the sample. A second low temperature relaxation is evident at lower PC contents indicating that some regions of this plasticized SPE are distinctly more mobile than others or perhaps, on this length scale, that some degree of phase separation is present. Activation energies for the mechanical :relaxation were also determined as a function of PC concentration and are significantly greater than those determined from conductivity measurements.