The local environment of arsenic atoms in vitreous samples of the system Sb2S3-As2S3-Tl2S has been studied by EXAFS at the AsK-edge. The crystalline compound Tl5.6As15S25.3 situated within the zone of glass formation of the Sb2S3-As2S3-Tl2S phase diagram was used as reference compound in order to derive appropriate phase and amplitude functions. The structure parameters determined were the number of first neighbours Nas-sl the arsenic-su sulphur distance, RAs-S, and the Debye-Waller factor, sigma(As-S). The influence of the glass-forming antimony sulphide Sb2S3 and the glass-modifying thallium sulphide Tl2S on the As2S3 host matrix has been shown.