The crystal structures of LaRu 4 P 12 and CeRu 4 P 12 are refined by the Rietveld analysis of the powder X-ray diffraction data. The bond distances and bond angles in both phosphides are obtained. La 1؊x Ce x Ru 4 P 12 (x ؍ 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.75, 0.8, 0.9, and 1) and CeOs 4 P 12 are prepared at high temperatures and high pressures. Powder X-ray diffractions of the alloys are studied at room temperature. The lattice parameters of the alloys do not change linearly with increasing x and markedly deviate from linear between x < 0.4 and x ؍ 0.6. The formal oxidation number of Ce atoms in the alloys is ؉3 below x ؍ 0.6. The electrical and magnetic properties of the alloys CeRu 4 P 12 and CeOs 4 P 12 are studied at low temperatures. The metal-toinsulator transition and magnetic ordering appear at around x ؍ 0.6. Above x ؍ 0.7 the alloys behave as a semiconductor. CeOs 4 P 12 exhibits a semiconducting behavior, with an activation energy of 0.034 eV. CeRu 4 P 12 and CeOs 4 P 12 show a very small, nearly temperature-independent susceptibility. Electrical and magnetic properties of the alloys, CeRu 4 P 12 and CeOs 4 P 12 are discussed.