A capillary electrophoresis method for the determination of diclofenac sodium in a commercial and simulated tablet formation has been described. The aim of this research was to develop a rapid, simple, efficient, and economical method to determine diclofenac sodium in pharmaceutials. The analysis was carried out in a bare silica capillary (75 pm i.d.) with a total length of 50 cm (28 cm to the detector) with a buffer solution containing 20 mM sodium tetraborate, pH 9.23. The applied voltage was 20 kV. Detection was achieved by ultraviolet absorption at 276 nm. Acetaminophen was used as an internal standard. The calibration curve was linear to the concentration range from 40.0 to 120.0 microg/mL with a correlation coefficient of 0.9992. The percentage recovery was found to be 103.12 +/- 0.91. The results showed that the method allows the determination of diclofenac sodium in commercially available pharmaceutical preparations.