The effect of carbon black and ceramic (α-Al2O3 and γ-Al2O3) particles on the cationic and polymer chain mobility have been investigated in poly(ethylene oxide)–lithium salt (LiClO4 and LiBF4) solid composite polymer electrolytes by using nuclear magnetic resonance (NMR) techniques. Proton (1H) and lithium (7Li) lineshapes and spin–lattice relaxation times were measured as a function of temperature. The 7Li data indicates that the lineshape and relaxation is affected by the coupling of its quadrupolar moment to the surrounding electric field gradients. Activation energies extracted from the 7Li relaxation data are in the range 0.20–0.25 eV. Results show that the addition of α-Al2O3 influenced both, the relative cation mobility and the polymer chain motion. Calculation shows that this result is in quantitative agreement with the conductivity data.