A CE method was developed for the simultaneous determination of Zr(IV) and Hf(IV) at trace levels. A lacunary Keggin-type [PW(11)O(39)1(7-) ligand reacted quantitatively with a mixture of trace amounts of Zr(IV) and Hf(IV) to form the so-called ternary Keggin-type anions [P((ZrW11)-W-IV)O-40](5-) and [P((HfW11)-W-IV)O-40](5-) in 0.010 M monochloroacetate buffer (pH 2.2). Since both ternary anions possessed different electrophoretic mobilities and high molar absorptivities in the UV region, Zr(IV) and Hf(IV) were determined simultaneously with direct UV detection at 258 nm. Each peak height was linearly dependent on the concentration of Zr(IV) or Hf(IV) in the range of 5.0 x 10(-7)-1.0 x 10(-5) M; a detection limit of 2 x 10(-7) M was achieved. The utility of the proposed CE method was demonstrated for the simultaneous determination of Zr(IV) and Hf(IV) in natural water samples with satisfactory results.