Abstract The development and application of a post-column detection system for K vitamins based on their photoreduction to the hydroquinone form is reported. The photoreduction yield is practically quantitative and occurs in a PTFE tubing coiled around a 6-W low-pressure mercury lamp in the presence of sodium dodecylsulfate and methanol. Factors affecting the rate of the photochemical reaction were optimised so that its contribution to the total broadening was negligible. The enhanced fluorescence and stability of the K vitamins reduced in micellar medium has permitted the use of a very sensitive photochemical detection system, which can work in aerobic conditions. Separations were carried out by reverse-phase chromatography using pure methanol as eluent. The determination of phylloquinone, menaquinone-4 and menadione in several real samples illustrates the potential of the photochemical detection system.