Thin films of In,S,, containing different quantity of sodium have been synthesized by annealing at 400degreesC of structures composed of thin indium, sulphur and indium fluoride layers sequentially evaporated on sodium-free glass substrates. The thin films obtained have been studied by X-ray diffraction (XRD). electronic microprobe analysis and X-ray photoelectron spectroscopy. They exhibit a beta-In2S3-like structure and the sodium atoms are homogeneously distributed in the whole films. In order to determine the structure of the material and particularly the position of the sodium atoms in the beta-In2S3 spinel matrix, single crystal of the same compound has been synthesized and then studied by XRD and transmission electron microscopy. These studies have shown that the materials obtained can be described by the general formulation [In-16](Oh)[In5.33-xNa3x rectangle(2.66-2x)](Td)S-32 (0 less than or equal to x less than or equal to 1.33). where Oh and Td, respectively, represent the octahedral and tetrahedral sites of the spinel structure. In thin films form. the maximum of sodium which can be introduced in the crystalline matrix has been shown to correspond to x = 0.9, which corresponds to the formula [In-16](Oh)[In4.4-Na2.7 rectangle(0.9)](Td)S-32. This new material family is called BINS. 2002 Elsevier Science B.V. All rights reserved.