Boron as boric acid was determined with chromotropic acid using a fluorescence detection—flow injection system. The flow system consisted of three streams, a carrier, a reagent and an alkaline solution. By mixing 0.5 M sodium hydroxide solution as the alkaline solution, the high background fluorescence of the reagent was diminished and the sensitivity was enhanced. The determinable concentration of boric acid was in the range 1 × 10 −8–1 × 10 −4 M. The detection limit was 5 × 10 −9 M of boron, corresponding to a signal-to-noise ratio of 3. The sample throughput was 60 h −1. The method was applied to the determination of boron as boric acid in water samples.