Analysis of the optical properties of bulk Cu (In1-x Gax) 3 Se5 mixed crystals synthesized from the elements as a function of the Ga content is presented. Measurements of the complex dielectric function ε (ω) = ε1 (ω) +i ε2 (ω) were performed at room temperature in the photon energy range of 0.8-4.7 eV using a variable angle of incidence ellipsometer. The spectral dependence of the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity were also derived. The structure observed in the dielectric functions attributed to the interband transitions E0, E1A, and E1B has been modeled using a modification of the Adachi's model. The results are in excellent agreement with the experimental data over the entire range of photon energies. The model parameters, including the energies corresponding to the lowest direct gap and higher critical points, have been determined using the simulated annealing algorithm. The values of E0 and E1A are found to increase linearly with the increasing Ga content.