The synthesis and structures of the three new compounds, KPbPS(1.84)Se(2.16) (1), RbPbPS(1.56)Se(2.43) (2), and CsPbPS(3.46)Se(0.54) (3), are reported. The solid state structures of 1-3 consist of two-dimensional layers of [PbP(S/Se)(4)] separated by alkali metal ions. The structure of 1 was solved in the orthorhombic space group Pna2(1). Compounds 2 and 3 possess the CsSmGeS(4) structure type, crystallizing in the orthorhombic space group P2(1)2(1)2(1). All compounds were refined as racemic twins. All chalcogen sites around the tetrahedrally coordinated P atoms show mixed S/Se occupancy; however, there is a preference for Se binding to Pb ions and S binding to alkali ions. A (31)P magic angle spinning NMR study on 1 suggests that, in mixed seleno-/thiophosphates, all of the anions [PS(x)Se(4-x)](3-) (x = 0, 1, 2, 3, 4) are present. The different amount of sulfur and selenium present in KPbPS(1.84)Se(2.16) (1), RbPbPS(1.56)Se(2.43) (2), and CsPbPS(3.46)Se(0.54) (3) is reflected in the solid state absorption spectra from which bandgaps of 2.2 eV were determined for 1 and 2, and a blue-shift to 2.5 eV was observed because of the higher sulfur-content in 3. Thermogravimetric analysis experiments indicated that, upon heating, compound 1 decomposes forming PbSe and sulfur together with other unidentified products. A Raman spectrum of compound 1 showed more bands than are usually observed in seleno- or thiophosphate salts and is another indicator of the mixed seleno-/thiophosphate anions found in 1.