The four series of quaternary sulfides RE3Zn0.5SiS7 (RE = La–Nd, Sm, Gd–Er), RE3Zn0.5GeS7 (RE = La–Nd, Sm, Gd–Er), RE3Cd0.5SiS7 (RE = La–Nd, Sm, Gd–Ho), and RE3Cd0.5GeS7 (RE = La–Nd, Sm, Gd, Tb) were prepared by reactions of the elements at 1050 °C. Analysis of their powder X-ray diffraction patterns indicates that they adopt the noncentrosymmetric hexagonal La3Mn0.5SiS7-type structure (space group P63, Z = 2). The structures of two Cd-containing members were refined from single-crystal X-ray diffraction data (Gd3Cd0.5SiS7, a = 9.9737(8) Å, c = 5.6287(4) Å; Gd3Cd0.5GeS7, a = 9.9739(3) Å, c = 5.7296(4) Å). La3Zn0.5GeS7 melts incongruently above 1300 °C. Ce3Zn0.5GeS7 shows simple paramagnetic behaviour arising from non-interacting Ce3+ species. Optical band gaps of 2.7 eV for La3Cd0.5SiS7, 2.3 eV for La3Cd0.5GeS7, and 1.8 eV for Ce3Zn0.5GeS7 were measured. Band structure calculations on La3Cd0.5SiS7 and La3Cd0.5GeS7 confirm the presence of band gaps whose magnitude is controlled by Si–S or Ge–S bonding interactions.