The bromide and chloride ion enhancement of chemiluminescence (CL) of trace metal ion catalyzed luminol oxidation by hydrogen peroxide was examined for chromlum(III), iron(II), and cobalt(II) ions. The enhancement remains nearly constant at different metal ion concentrations but It varies with the pH of the luminol-H202 solution. In the presence of 0.3 M Br, an increase of 6.2-, 3.5-, and 1.4-fold of CL signal was observed for Cr(III), Fe(II), and Co(II), respectively, at the optimized pH of analysis. The feasibility of analytical applications for the halide effect Is discussed. A general mechanism for the halide effect is proposed.