High-pressure electrical conductivity studies have been carried out with poly( p-phenylene)s with oxyethylene side-chains (PPP(EO)(x/y)), which were blended with LiCF3SO3. Measurements were performed at pressures up to 280 MPa and at different temperatures. The influences of salt concentration, side-chain length, temperature, and plasticizer content on the relative conductance and activation volume are investigated. The temperature-dependent conductivity of the sample is non-Arrhenius and exhibits Williams-Landel-Ferry (WLF) behavior. The logarithm of relative conductance for PPP(EO)(x/y)/LiCF3SO3 decreases almost linearly with increasing pressure but increases with salt concentration and side-chain length. As temperature increases, the activation volume becomes smaller but remains positive for PPP(EO)(x/y)/LiCF3SO3. At higher salt concentrations and longer side-chain lengths, a smaller activation volume for the ion motion is found. These results can be interpreted such that PPP(EO)(x/y)/LiCF3SO3 behaves like a true polymer electrolyte where ion transport is mediated by segmental motions of the EO side-chains. The addition of tetraethylene glycol dimethyl ether (TEGDME) as a plasticizer increases the activation volume but reduces the conductance.