The article describes a flow injection chemiluminescence (CL) assay for catechol. It is based on the catechol-induced reduction of the CL of a system composed of fluorescein, hexacyanoferrate(III) and silver nanoparticles in strongly alkaline solution. Under the selected conditions, the decrease in CL intensity is proportional to the concentration of catechol in the 0.1 to 10 μM range. The detection limit is 5.0 nM, and the relative standard deviation is 1.5 % (for n = 11 at a 1.0 μM concentration of catechol). The method was successfully applied to the determination of catechol in spiked environmental water samples.