To clarify the solid solution regions of CuIn3(SxSe1 (-) (x))(5) and CuGa3(SxSe(1) (-) (x))(5) systems and their optical properties, we prepared Culn(3)(S,Se)(5) and CuGa3(S,Se)(5) samples by a mechanochemical process and post-heating. Single-phase solid solutions with a tetragonal stannite-type structure could not be obtained for Culn(3)(SxSe1 (-) (x))(5) with 0 <= x < 0.1. On the other hand, we successfully obtained single-phase solid solutions with a tetragonal stannite-type structure for CuGa3(SxSe1 (-) (x))(5) with 0.0 <= x <= 1.0. The solid solution region of the CuGa3(SxSe1 - x)(5) system is much wider than that of the Culn(3)(SxSe1 - x)(5) system. The band gap energy of the CuGa3(SxSe1 (-) (x))(5) solid solution linearly increased from 1.85 eV of CuGa3Se5 (x = 0.0) to 2.58 eV of CuGa3S5 (x = 1.0). The energy levels of the valence band maxima (VBMs) were estimated from the ionization energies measured by photoemission yield spectroscopy (PYS). The ionization energy of stannite-type CuGa3Se5 (5.69 eV) is approximately equal to that of CuIn3Se5 (5.65 eV). The energy levels of the VBMs of the CuGa3(SxSe1 - (x))5 solid solution decrease with increasing S content, x = S/(S + Se) ratio. The conduction band minimum (CBM) levels of CuGa3(SxSe1 - x)(5) are almost constant with x = S/(S + Se) ratio. CuIn3Se5, CuGa3Se(5), CuGa3S5 and CuGa3(S,Se)(5) solid solution are expected to be useful for controlling the valence band offset (Delta E-V) and the conduction band offset (Delta E-c) at the interface between buffer layer and absorber layer in CIGS solar cells. (C) 2017 Published by Elsevier B.V.