A stereospecific capillary electrophoresis assay for the simultaneous determination of related substances and the enantiomeric purity of escitalopram was developed by a central composite face-centered factorial design and subsequently validated. Separations were carried out in a 50 microm, 47/40 cm fused-silica capillary. The optimized conditions included 20mM phosphate buffer, pH 2.5, containing 0.5mg/ml beta-cyclodextrin and 22 mg/ml sulfated beta-cyclodextrin as background electrolyte, an applied voltage of -20 kV and a temperature of 28 degrees C. Salicylic acid was used as internal standard. The assay was validated for the (R)-enantiomer of citalopram and the enantiomers of the impurity citadiol in the range of 2.5-150 microg/ml and 2.5-50 microg/ml, respectively. The limit of detection was 0.02% for all compounds, the limit of quantitation 0.05%, relative to a concentration of escitalopram of 5mg/ml. Intraday precision of migration time and peak area ratio were in the range of 0.17-0.44% and 1.64% and 6.25%, respectively. Relative standard deviations of interday precision ranged between 0.84% and 1.85% in the case of migration times and between 5.20% and 9.28% for peak area ratio. The assay was applied to the determination of the purity of escitalopram in bulk drug and tablets. (R)-Citalopram and (S)-citadiol were detected as impurities.