Structural, electronic and optical properties of CdSxSe 1-x at various compositions (0 ≤ x ≤ 1) are investigated using full potential linearized augmented plane waves (FP-LAPWs) method. A structural phase transition from zinc-blende to wurtzite is observed in CdSxSe1-x with increase in S concentration. The theoretically calculated phase transition as well as structural parameters like ground state energies, lattice constants and bulk moduli is in agreement with the experimental results. As CdSxSe1-x compound belongs to strongly correlated systems, therefore modified Becke-Johnson potential is used to calculate band gap energies and optical properties of the compound. The direct band gap nature of the material varies in the visible part of the spectral region for the whole range of S. The optical properties, like dielectric functions, refractive index, birefringence and energy loss function, are also calculated. The hexagonal wurtzite CdSSe is anisotropic and the calculated birefringence is found to be positive in the lower energy levels and negative in the higher levels. © 2013 Elsevier B.V. All rights reserved.