The synthesis, characterization and application of 2-(2-hydroxybenzylidenimine)benzene-arsonic acid (HBBA) as a reagent for the fluorimetric determination of boron are described. This reagent reacts with boric acid in 85% w/w sulphuric acid medium to yield a fluorescent compound. The reagent is not fluorescent in those conditions. Heating at 90 degrees for 45 min is needed for the compound to be formed. The linear calibration range is 0.1-8 microg/ml in the solution measured. The detection limit of the method is 0.01 microg/ml. The method has been applied to determine boron in vegetal material.