Upon irradiation with short-wavelength UV light the tranquillizer demoxepam is converted into a highly fluorescent product. This reaction serves as a sensitive and selective means of detection of low levels of demoxepam in e.g., serum after separation by means of reversed-phase high-performance liquid chromatography. The effect of mobile phase composition and time of irradiation on the intensity of the fluorescence signal has been studied. A residence time of about 2 min in the post-column photochemical reactor is optimal, band broadening being efficiently suppressed (delta t ca. 1 sec) by means of air segmentation. Linear calibration graphs are obtained over a three-orders of magnitude concentration range; the detection limit for demoxepam is about 100 pg. Further work has demonstrated that detection limits of between 40 and 100 pg can also be obtained for the photoproducts of the phenothiazines fenergan, largactil, levopromazine and nedaltran. The calibration graphs show good linearity and the analysis of spiked serum samples was successful.