A quick, simple, sensitive, efficient and stability-indicating reverse-phase ultraperformance liquid chromatographic method for the estimation of propylparaben, methylparaben and sodium benzoate in a pharmaceutical liquid oral formulation was developed. A Waters Acquity UPLC BEH C₁₈, 50 × 2.1 mm, 1.7 μm i.d. column was used to perform chromatographic separation with a 0.1% perchloric acid mobile phase used as solvent A and a mixture of 0.1 % perchloric acid and methanol in the ratio 20:80 (v/v), respectively, as solvent B. The experiments were carried out at a flow rate of 0.4 ml/min and the detection wavelength was 240 nm. The compartment temperature of the column was set at 40°C and the injection volume was set at 2 μl. The main aim of the research was to develop a single UPLC assay method for promethazine (active ingredient) and preservatives in the oral solution of promethazine HCl and dextromethorphan HBr that contains promethazine (active ingredient) and methylparaben, propylparaben and sodium benzoate (preservatives). An assay of dextromethorphan HBr was developed and validated by another HPLC method. The drug and preservatives were eluted at retention times of 19.3 min for promethazine HCl, 9.3 min for methylparaben, 18.9 min for propylparaben and 8.9 min for sodium benzoate. Validation of the developed method was carried out as stated by the International Conference on Harmonization guidelines ICH Q2B and under USP<1225>. The analytical parameters verified specificity/selectivity, linearity, accuracy, ruggedness and robustness. The linearity ranges of promethazine HCL, methylparaben, propylparaben and sodium benzoate were 10-100, 10-80, 1.0-8.0 and 10-80 μg/ml, respectively, with a correlation coefficient of active ingredients and preservatives of 1.00. Percentage recoveries of promethazine, propylparaben, methylparaben, and sodium benzoate were 100.0-100.2, 99.0-100.3, 99.5-98.0 and 99.0-100.0%. The validated analytical method proves that the method is specific, precise, linear, accurate, sensitive, rugged and stable, indicating the quantification of the active ingredient and all preservatives in liquid oral formulations.