The compounds Ln(2)O(2)S (Ln = Sm, Eu) were prepared from the respective oxides by partial sulfurization under flowing CS2/Ar/O-2 atmosphere. The main feature of the structure is the packing of S2- and O2- anions with Ln(3+) located in the center of a capped octahedron. Electrical conductivity measurements on pressed pellets showed that both phases are highly resistive with conductivities lower than 10(-8) S cm(-1) at room temperature. The optical bandgaps were assessed at 4.2 and 4.4 eV for Sm2O2S and Eu2O2S, respectively. Electronic structure calculations were performed by means of extended Mickel tight-binding method. The insulating properties are in good agreement with the results predicted by theoretical calculations. (C) 2002 Elsevier Science Ltd. All rights reserved.