Powder X-ray diffraction and optical studies were performed to characterize the fundamental structural aspects and determine the energy band gap E-g of several compositions of the Cu(In1-xGax)(3)Se-5 semiconducting system. In contrast to what has been published for thin films, the present work, carried out on synthesized bulk samples, indicates that a solid solution is formed over the entire range of composition. The parameters a and c of the tetragonal chalcopyrite-related unit cell were found to vary from 5.7541(5) and 11.538(3) Angstrom for x = 0 to 5.4995(8) and 10.946(3) Angstrom for x = 1, when In is continuously replaced by the smaller Ga atom. The variation of the energy band gap with composition in this system can be expressed better by a quadratic relations of the form E-g = a + bx + cx(2) than by a straight line as has been reported by other authors.