The luminescent system H3BO3 / 2 - hydroxyl - 4 - methoxy - 4'chloro - benzophenone (HMCB) / H2SO4 conc. was first studied by the method of the continuous variations to identify the complexes A1R2 and A2R2. Then further methods, based on the alternative variations of the ligand and the central atom, gave a more detailed view of the system and identified the complexes A1R1, A1R2 et A2R2 and their apparent global constants (β(ij) ) and apparent efficiency coefficients (ϕ(ij)). Also, a solid complex of the type A2R2 was isolated and analyzed, presenting a central hexagonal ring with the atoms: B (2x), S (1x) and O (3x), bound to two reagents (HMCB). A scheme of reactions could be established and the hypothesis of Langmyhr et Arnesen(32) confirmed, that H3BO3 can form in H2SO4 conc. with certain reagents, complexes of a A2R2 type similar to the one identified here. To be able to apply the fluorimetric HMCB method to determine traces of B in H2SO4 conc. in water, soils and plants it was necessary to eliminate the organic matter and traces of the residual fluorescence. A mineralization with addition of drops of H2O2 (30%), during the heating, to 3 ml of H2SO4 conc. was so developed. The interferences with metallic ions is negligible, the one of F- is low and that of NO3- and H2O2 can be canceled. So with this method it was possible to determine nanograms of boron in numerous very small samples of water, soil extracts, humus and plants with an relative errors (E%) ≤ to 10%. It is of outstanding interest that with the application of this method to the solid and light humus extracts of different French soils, obtained by extractions and concentrations according to Hardisson et al., quantities of B of the order of 10 ppm were found. These humus fractions strongly bound to boron, may well be an essential characteristic of soils.