The optical and electrical properties of the new [In-16](Oh)[In(5.33-x)Na(3x)square(2.66-2x)](Td)S-32 (BINS) thin films have been studied. The determination of the optical band gap of films containing different rates of sodium has shown that it linearly increases from 2.10eV, when the films are pure beta-In2S3, to 2.95eV, when their sodium content corresponds to x = 0.9 i.e. [In-16](Oh)[In(4.4)Na(2.7)square(0.9)](Td)S-32 compound. This evolution of the band gap has been discussed in terms of In-S bond length and electronegativity of Na and In elements. All the BINS thin films have an n-type electrical conductivity, which decreases from 4 x 10(-7) to 10(-7) S cm(-1) when x increases from 0.05 to 0.9, respectively. The electrical conductivity of pure beta-In2S3 has been found around 2 x 10(-8) S cm(-1), which is lower than that of films containing sodium. The variation of the room conductivity of the films is related to the sodium distribution in the crystalline structure. (C) 2002 Elsevier Science B.V. All rights reserved.