We synthesized solid solutions of β-CuGaO 2 and β-LiGaO 2 (i.e., β-(Cu 1-x Li x )GaO 2 ) by partial ion exchange of Cu + in β-CuGaO 2 with Li + from LiCl in the composition range of 0 ≤ x ≤ 0.89. The energy band gap of β-CuGaO 2 (1.47 eV) increased linearly up to 3.0 eV with increasing Li content, covering the full visible range. The crystal structures of the solid solutions were analyzed using the Rietveld method. The structural distortions of the solid solutions with respect to the ideal binary wurtzite-type structure were relatively small because of the similar ionic radii of Li + , Cu + , and Ga 3+ . Based on a recently proposed hypothesis relating structural distortion to the nature of the band gap (i.e., direct or indirect), it is expected that the solid solution has a direct band gap. We anticipate that this solid solution system will contribute to the realization of oxide-based optoelectronic devices.