The chemical stability of perphenazine in two commercially available vehicles for oral liquid dosage forms was studied by using a stability-indicating high-performance liquid chromatographic assay method that was developed in our laboratory. The results indicate that the recommended beyond-use date of 180 days in Ora-Sweet syrup vehicle (final pH of the dosage form was 4.2) is not valid for this product. However, in Humco's simple syrup (final pH of the dosage form was 4.5), perphenazine was stable for at least 183 days. The decomposition of perphenazine followed first order law. The decomposition constant at room temperature in Ora-Sweet syrup vehicle was 2.3 times higher (0.0007 per day) versus in Hucmco's simple syrup (0.0003 per day). The pH values of both dosage forms remained constant, and the physical appearance of both dosage forms did not change during this study period. For a stable oral liquid dosage form did not change during this study period. For a stable oral liquid dosage form of perphenazine, the pH value is very critical.