The separation and simultaneous determination of caffeine, paracetamol, and acetylsalicylic acid in two analgesic tablet formulations was investigated by capillary electrochromatography (CEC). The effect of mobile phase composition on the separation and peak efficiency of the three analytes was studied and evaluated; in particular, the influence of buffer type, buffer pH, and acetonitrile content of the mobile phase was investigated. The analyses were carried out under optimized separation conditions, using a full-packed silica capillary (75 mum ID; 30.0 cm and 21.5 cm total and effective lengths, respectively) with a 5 mum C-8 stationary phase. A mixture of 25 mm ammonium formate at pH 3.0 and acetonitrile (30:70 v/v) was used as the mobile phase. UV detection was at 210 nm. Good linearity was found in the range of 50-200, 20-160, and 4-20 mug/mL for acetylsalicylic acid (r(2) = 0.9988), paracetamol (r(2) = 0.9990) and caffeine (r(2) = 0.9990), respectively. Intermediate precision (RSD interday) as low as 0.1-0.8% was found for retention times, while the RSD values for the peak area ratios (A(analyte)/A(IS)) were in the range of 1.9-2.9%. The optimized CEC method was applied to the analysis of the studied compounds present in commercial tablets.