The potential of CZE with LIF detection in the separation and determination of low-molecular mass aldehydes involving precolumn derivatization with fluorescein 5-thiosemicarbazide was investigated. Different variables that affect derivatization (pH, fluorescein 5-thiosemicarbazide concentration, time and temperature) and separation (pH and concentration of the BGE, kind and concentration of surfactants at levels higher and lower than CMC, and applied voltage) were studied. The separation was conducted within 16 min by using borate buffer (60 mM; pH 10) with 10) mu M polyethylene glycol tert-octylphenyl ether as modifier. Good linearity relationships (correlation coefficients ranged from 0.9978 to 0.9994 for aldehydes) were obtained between the peak areas and concentration of the analytes (0.5-100 mu g/L). The LODs for aldehydes were achieved at submicrogram-per-liter level (0.15-0.35 mu g/L), which indicated that the proposed method surpassed other electrophoretric alternatives in terms of LOD, in many cases even at ca. 1000-fold. The inter-day precision (RSD, %) of the aldehydes ranged from 5.2 to 8.3%. Finally, the method was successfully applied to bottled drinking-water samples, and the aldehydes were readily detected at 0.6-4.4 mu g/L levels with average recoveries ranging from 99.1 to 103.5%.