Dissociation constants (pK(a)s) of acidic, basic, and amphoteric pharmaceuticals were exactly determined by capillary electrophoresis. A general equation for use in calculation of pK(a)s from solute mobilities observed at different pHs was derived to be suitable for a multivalent compound with close pK(a)s such as angiotensins, which are bioactive octa- or decapeptides. The obtained pK(a) values were highly consistent with the values determined by conventional methods. For verapamil, the detection limit was 9.8 mu M (49 fmol), the total analysis time was 12 min, and the relative standard deviation of the obtained pK(a) value was 0.11%. This method was also applicable to a mixture of two components, e.g., a bioactive compound and the prodrug, because they have different mobilities and are separated from each other.