The stopped-flow mixing technique was used to develop an automated room-temperature liquid phosphorimetric method for the straightforward, fast determination of naproxen in serum samples. The interaction of naproxen with thallium(I) ions in the presence of sodium dodecyl sulfate allows one to obtain a kinetic curve that levels off within only 2 s. This approach allows the development of very fast kinetic and equilibrium methods by using the slope and amplitude of the kinetic curve, which are directly proportional to the naproxen concentration, as analytical parameters. The linear calibration range thus achieved is 0.06-15 or 0.08-15 μg mL−1, depending on wheather slope or amplitude measurements are used. The within- and between-assay precision data, as well as the selectivity of the method are more than adequate for the determination of naproxen in serum samples pretreated with acetonitrile. Analytical recoveries range from 90.7 to 104.0%.