Two polytypes of potassium rare-earth-metal hexaselenodiphosphates(IV), K(RE)P2Se6 (RE = Y, La, Ce, Pr, Gd), have been synthesized from the stoichiometric reaction of RE, P, Se, and K2Se4 at 750 degrees C. Both single-crystal and powder X-ray diffraction analyses showed that the structures of these polytypes vary with the size of the rare earth metals. For the smaller rare-earth metals, Y and Gd, K(RE)P2Se6 crystallized in the orthorhombic space group P2(1)2(1)2(1). The yttrium compound was studied by single-crystal X-ray diffraction with the cell parameters a = 6.7366(5) Angstrom, b = 7.4286(6) Angstrom, c = 21.603(2) Angstrom, and Z = 4. This structure type comprises a layered, square network of yttrium atoms that are bound to four distinct [P2Se6](4-) units through selenium bonding. Each [P2Se6](4-) unit possesses a Se atom that is not bound to any Y atom but is pointing out into the interlayer spacing, into an environment of potassium cations. For larger rare-earth metals, La, Ce, and Pr, K(RE)P2Se6 crystallized in a second, monoclinic polytype, the structure of which has been published. Both of these two different polytypes can be related to each other and several other isoelectronic chalcophosphate structures based on a Parthe valence electron concentration analysis. These structures include Ag4P2S6, K2FeP2S6, and the hexagonal M(II)PS(3) structure types. The magnetic susceptibilities of the title compounds have been studied, and the behavior can been explained based on a simple set of unpaired f-electrons. The diffuse reflectance spectroscopy also showed that these yellow plates are moderately wide band gap (similar to 2.75 eV) semiconductors.