Cannabidiol reacts with 2,4-dinitrophenol in alkaline medium to give a red colour having maximum absorbance at 500 nm. The reaction is specific for cannabidiol with 0.1 μg/10 ml as a minimum limit of detection. It also provides the basis for a new spectrophotometric determination of pure cannabidiol and in the presence of other organic compounds. The maximum tolerable limits of these are reported. The method is successfully applied to the determination of cannabidiol from leaves ofCannabis sativagrowing wild and cultivated in Pakistan.