The pH-dependent surface charge densities of the acetylated and the native purple membrane were determined by the ESR spin lable method. The spin probe is CAT₁₂. The number of surface charges shieded by acetylation was adapted as a criterion to calculate the surface charge numbers on both sides of the purple membrane from surface pH 4─11. The result shows that the total surface negative charge numbers are 9 per bacteriorhodopsin at surface pH 5─9 but increases both above surface pH 9 and below surface pH 5. It supports strongly the model based on five divalent cation binding sites on the surface of purple membrane.