Background: Presumptive colour tests are commonly used in forensic drug analysis for the preliminary screening of suspected illicit substances. The Chen–Kao reaction may assist in the preliminary examination of certain synthetic cathinones, although its specificity for individual compounds is limited.Objective: To observe and document the colourimetric reaction of a standard mephedrone sample with the Chen–Kao reagent.Methods: The standard mephedrone sample was examined for colour, texture, odour, physical appearance, and solubility. The sample was subsequently subjected to the Chen–Kao reagent system comprising acetic acid, copper sulphate solution, and sodium hydroxide solution. The resulting colour changes and precipitate formation were recorded visually.Results: The preliminary examination recorded the sample as white or off-white to brown, crystalline in texture, and in powder form, with no odour or a slight chemical-like smell. The sample was reported to be readily soluble in water and ethanol. During the colourimetric reaction, the observed sequence was blue, followed by blue-green, and finally an orange-brown precipitate in the centre of the spot plate.Conclusion: The Chen–Kao reaction produced a visible colourimetric response with the standard mephedrone sample under the experimental conditions. However, the reaction cannot independently confirm the identity of mephedrone because of its limited specificity.