Photoluminescence (PL) and optical absorption (OA) measurements have been used to study CuInS2xSe2(1-x) alloy at temperatures down to 4.2 K for x from 0 to 1. Exciton related PL lines were observed for all studied elemental compositions. For x=0 the A and B excitons were resolved in the PL spectra. Dependencies of the optical band gaps of the alloy on the elemental composition were derived from both the PL and OA spectra. The band gap was found to be changing nonlinearly with respect to x. The derived bowing coefficient b = 0.13 eV. The possible nature of the observed luminescence centres and radiative recombination mechanisms is discussed. (C) 2003 Elsevier Science B.V. All rights reserved.