A capillary zone electrophoresis method for the separation and determination of thiamin and its phosphate esters (free thiamin, thiamin monophosphate, and thiamin pyrophosphate) was developed and optimized. The efficiency achieved with boric acid-sodium tetraborate decahydrate buffer (pH 8.24; 65-8 mM) at an applied potential of 30 kV gave the detection limit (S/N = 3) and the limit of quantitative measurement (S/N = 10) of thiamin and its phosphate esters ranging from 10(-4) to 6 x 10(-4) mM and from 6 x 10(-4) to 1.2 x 10(-3) mM, respectively. The effects of pH on separation and migration times of thiamin and its phosphate esters are described. The method was validated and applied to the quantitative determination of thiamin in commercial tablets containing both a massive and a normal dose of thiamin.