Capillary zone electrophoresis was optimized to quantitatively determine codeine and paracetamol via central composite factorial design. Critical parameters (concentration, buffer, pH, voltage) assessed effects on resolution, analysis time and efficiencies. Optimum separation conditions were achieved using phosphate buffer 20 mM (pH 6.8) and voltage (15 kV). The optimized procedure easily determined codeine and paracetamol with separation in less than 3 min. Calibration curves (R > 0.999) were prepared, with LODs of 13.5 and 340 ng mL(-1) for codeine and paracetamol, respectively, and a good R.S.D.% (<3%). This method was applied to determine codeine and paracetamol in pharmaceutical formulations; recoveries coincided with stated contents.