The condensation reactions of Se(IV) with 3,4-diaminobenzoic acid and 4-bromo-1,2-phenylenediamine have been studied by means of UV spectra and kinetic investigations. A mechanism for the formation of 2,1,3-benzoselenadiazole in acidic medium is proposed. The influence of substitution at C(4) in 1,2-phenylenediamine on the reactivity of the system is discussed.