A stable oral liquid dosage form of the easily oxidizable drug captopril was developed.The effect of ionic strength, initial drug concentration, buffer concentration, buffer species, cosolvents, and chelating agent on the stability of captopril were studied.The cosolvents, chelating agent, and higher captopril concentrations improved the stability of captopril in aqueous systems.The excipients protected the thiol group of captopril from oxidation and extended its shelf-life in aqueous systems.