Using hydrothermal processing, we prepared cerium-bismuth solid solution oxide powder ((Ce-0.73, Bi-0.27)O2-delta, CBO) with brownish yellow color, a fluorite-type crystal structure, and decomposition activity on 2-naphthol in water at 50-75 degrees C in the dark. The contribution of the Mars - van Krevelen (MvK) mechanism was inferred from the activation energy and the valence change of Ce ion before and after decomposition. Photocatalysis by visible light illumination increased the CBO decomposition activity. Modification of MnOx onto the CBO provided higher decomposition activity on 2-naphthol in the dark or under visible light. The activation energy and the valence difference indicated that the dark activity was imparted by the MvK mechanism of MnOx. A contribution by oxygen diffusion in CBO was inferred as enhancing the MvK mechanism. The decomposition activity was obtained even at 30 degrees C. A synergistic effect achieved by the combination of CBO with MnOx was inferred from calculation of the reaction constants. Results also show that MnOx modification extended the CBO photon absorption range.