In this paper a simple, selective and inexpensive kinetic method was developed for the determination of morphine based on its inhibitory effect on the Janus green–bromate system in sulfuric acid media. The reaction was monitored spectrophotometrically at 618nm by a fixed time method. The effect of different parameters such as concentration of reactants and temperature on the rate of reaction was investigated and optimum conditions were obtained. The calibration curve was linear in the concentration range 0.07–7.98mgL⁻¹ of morphine, and detection limit of the method was 3.0×10⁻²mgL⁻¹. The relative standard deviation for five determinations of 3.74mgL⁻¹ of morphine was 0.57%. Finally, the proposed method was successfully applied to the determination of morphine in human urine and serum as real samples.