The chemical model of boric acid was built by means of the Modified Bromley Methodology using mean activity coefficient data and stoichiometric formation constant values in different ionic media at 25 degreesC. This study was performed considering the hydrolysis equilibrium of boric acid in aqueous solution, obtaining the thermodynamic equilibrium constant (log(0) K = 9.23 +/- 0.02) and the corresponding interaction parameters. Taking into account the chemical isomorphism between H(2)ASO(3)(-) and H2BO3-, the extension of the model to natural waters was compared with the chemical model of arsenite anion. The ionic radii for both species as well as all the ionic interaction parameters of these anions with the main cations in natural waters (Ca2+, Mg2+ and Sr2+) and Li+ were estimated. The model was validated against (log, K versus I) data in LiBr medium by using the predicted B-Li+,B-H2BO3-value, obtaining a sigma (log(1) K) = +/-0.02 with the four available log(I) K data. (C) 2003 Elsevier Science B.V. All rights reserved.