The preparation of new poly(p-phenylene)s with oligo(oxyethylene) side chains is reported. The solid state structure of the polymers as well as that of their mixtures with either lithium triflate or lithium bis(trifluoromethanesulfone)imide is described as a molecular composite consisting of rigid main chains embedded in a matrix of side-chain segments or side-chain/salt mixtures. DSC and temperature-dependent X-ray scattering prove that the side-chain matrix in the pure polymers and polymer/salt mixtures is completely amorphous. Room-temperature conductivities of more than 10(-6) S/cm are obtained in some of these solid polymer electrolytes. Plasticization of the materials with oligo(ethyleneglycol)-dimethylether (OEGDME) improves the conductivity up to 5.10(-5) S/cm. The stiff polymer backbone induces a high mechanical plateau-modulus in the temperature range between 0 and 90 degrees C, which is of considerable importance for potential applications of these molecular composites as separators in rechargeable solid state lithium cells.