The reaction of Ce or Gd with a molten mixture of A(2)Se/P2Se5/Se (A = Rb, Cs) produced the quaternary compounds Rb3CeP2Se8 (I) and Cs3GdP2Se8 (II), as well as Rb2CeP2Se7 (III) and Rb2GdP2Se7 (IV). The orange crystals of I, II and the red crystals of III, IV are air- and water-sensitive. Compounds I, II crystallize in the monoclinic space group P2(1)/c, and for I, a = 9.6013(2) Angstrom, b = 18.0603(1) Angstrom, c = 10.0931(1) Angstrom, beta = 90.619(1)degrees, and Z = 4. Compounds III, IV crystallize in the monoclinic space group P2(1)/n, and for IV, a = 10.137(2) Angstrom, b = 7.212(1) Angstrom, c = 20.299(2) Angstrom, beta = 98.23(1)degrees, and Z = 4. Compounds I, II have a 1-D structure with [(RE)P2Se8](n)(3n-) chains separated by A(+) cations. The RE3+ lanthanide cation is in a bicapped trigonal prismatic coordination with four tridentate [PSe4](3-) ligands. Compounds III, IV possess a 2-D structure. Each layer consists of [RE(PSe4)](x) "chains" interstitched in two dimensions by hexadentate [P2Se6](4-) ligands. The trivalent RE cation is in a square antiprismatic coordination. The compounds were characterized with differential thermal analysis, far-IR, and solid-state UV/vis diffuse reflectance spectroscopy. Magnetic measurements indicate that the compounds obey the Curie law with mu(eff) values close to those of the free RE3+ ions.