The influence of buffer pH and electrolyte modifier on the migration behavior and separation of thirteen sulfonamides was investigated by capillary zone electrophoresis. The results indicate that precise optimization of buffer pH is crucial in improving the separation of some closely migrating sulfonamides. On the addition of either an appropriate amount of an organic modifier (methanol or acetonitrile) or a low concentration of beta-cyclodextrin (beta-CD, 0.5 mM) to a phosphate-borate buffer at pH 6.85 and an applied voltage of 20 kV, the resolution of peaks between sulfathiazole and sulfamethoxypyridarine is markedly enhanced and effective separations of thirteen sulfonamides are achieved within a relatively short time. Methanol gives better resolution than acetonitrile as an organic modifier. Weak inclusion complexation occurs between beta-CD and sulfonamides, with the exception of sulfathiazole. The formation constants of thirteen sulfonamides with beta-CD are reported.