We herein report the synthesis of a series of Bi-based oxyhalides Bi4MO8X (M = Nb, Ta; X = Cl, Br) via solid-state reaction at different calcination temperatures and using various precursor ratios to improve their photocatalytic activity for water oxidation under visible light. The calcination temperature employed gave an influence on the crystallite sizes, morphologies, specific surface areas, and surface compositions. Interestingly, when stoichiometric precursors were heated at higher temperatures, the crystallite sizes of Bi4MO8X decreased with cleavage occurring along in-plane direction. Elemental analysis indicated that the unusual trend in Bi4MO8X can be explained by the volatile nature of halogen at higher temperatures. The volatilization of the halogen species was minimized when an excess of the halogen precursor BiOX was employed. The Bi4MO8X samples prepared with excess BiOX exhibited considerably higher photocatalytic activities for water oxidation under visible light, due to the suppressed halogen defects at which recombination of photogenerated carriers can occur. These findings offer a useful insight into the preparation of active oxyhalide photocatalysts.