Based on the formation of a Keggin-type [PMo12O40](3-) complex, a sensitive capillary electrophoresis (CE) method was developed for the determination of PM with direct UV detection at 220 nm. A mixture of alpha- and beta -Keggin-type [PMo12O40](3-) complexes was readily formed in a sample solution consisting of a trace amount of P(V), 2.5 mm Mo(VI), 0.050 M p-C6H3(CH3)-2-SO3H (XSA), and 60% v/v CH3CN. When a 0.05 m HCl and 60% v/v CH3CN solution was used as a migration electrolyte, the Keggin complexes exhibited a sharp and well-defined peak in the electropherogram. The peak area was linearly dependent on the PM concentration in the range of 5 X 10(-7)-5 x 10(-5) m; a detection limit of 1 x 10(-7) m was achieved. In comparison with indirect UV detection, the direct UV detection is about ten times more sensitive, because the Keggin complexes possess high molar absorptivities. The developed CE method was applied to the determination of PM in river water, and the results were in good agreement with those obtained by ion chromatography (IC) and colorimetry (COL) based on the formation of mixed-valence heteropoly blue species.