A MEKC method has been developed for the simultaneous determination of guaifenesin (GF), pseudoephedrine (PE), and dextromethorphan (DM) in capsule dosage form using 50 mM sodium dodecyl sulfate (SDS) added to a pH 8.5 100mM Tris run buffer at a voltage of 25 kV. The method utilized UV detection at 220nm and a 72 cm × 50 μm i.d. uncoated fused-silica capillary. The detection limits for GF, PE and DM were 1000, 500, and 500 ng/mL, respectively for a 5 s injection. The calibration curves were linear over ranges of 100–1000 μg/mL, 15–150 μg/mL and 5–50 μg/mL for guaifenesin, pseudoephedrine and dextro-methorphan, respectively, with sodium benzoate (SB) as internal standard. Coefficients of determination were greater than 0.9989 (n = 12). Precision and accuracy of the method were 0.60–2.18% and 0.44–4.20%, respectively for GF, 1.62–3.15% and 0.33–2.98%, respectively for PE, and 0.19–4.36% and 1.77–6.92%, respectively for DM. In comparison to high-performance liquid chromatography, the MEKC method is simple, capillary columns are low-cost, and there is high-resolution efficiency with a minimal consumption of environmentally friendly chemicals.