Based on the self-consistent ab initio full potential-linearized augmented plane wave method, the structural, electronic, optical, and thermodynamic properties of CdSe (x) Te1-x ternary semiconductor alloys have been investigated. The exchange-correlation potential was calculated using both the generalized gradient approximation (GGA) by Perdew-Burke-Ernzerhof (PBE) and the GGA by Engel-Vosko (EV-GGA). The ground-state properties are determined for the cubic bulk materials CdSe, CdTe, and their mixed crystals at various concentrations (x = 0.25, 0.5, and 0.75). Deviation of the lattice parameter from Vegard's law and the bulk modulus from linear concentration dependence has been examined. The microscopic origins of the band-gap bowing parameter have been discussed. Moreover, the refractive index and the optical dielectric constant for CdSe (x) Te1-x are studied using different models. Besides, the thermodynamic stability of the alloys of interest is investigated by means of the miscibility critical temperature.