Abstract: Bismuth oxyiodide plates were prepared by a solution -based method and then calcined at various temperatures. The as-prepared samples were characterized with X-ray diffraction (XRD), thermogravimetry (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV-Vis diffuse reflectance spectra (DRS), respectively. The photocatalytic activity of the samples was evaluated by photocatalytic decolorization of a methylene blue under visible light (λ > 420 nm) irradiation. Results show that BiOI is not a thermally stable compound. It undergoes a series of transformation BiOI→Bi₅O₇I→αBi₂O₃ during thermal treatment. The band gaps of the as-prepared samples were found to be 1.80, 2.47, 2.77, and 3.15 eV for BiOI, Bi 5O7I, Bi₅O₇I/ Bi₂O₃ mixture, and Bi₂O₃, respectively. Under visible light irradiation, the photocatalytic activity follows the order: BiOI > Bi₅O₇I > Bi₅O₇I/Bi₂O₃ mixture > Bi₂O₃. This corresponds to the absorption ability of the catalysts to visible light.