Illumination of ruthenium-catalyzed Belousov-Zhabotinsky reaction decreases the rotational frequency of spirals at low bromate concentrations but increases the frequency at high bromate concentrations. The effective diffusion coefficient D deduced from the Keener-Tyson relation for the spirals, D approximate to omega/3k(2), is independent of light intensity (D = 2.5 x 10(-6) cm(2)/s).