Sulfuric acid has many industrial applications. Its main use can be related to fertilizer manufacture, process with ore, chemical synthesis, liquid effluents treatment and the petroleum refine. A main characteristic of this acid is its behavior depending on concentration. When it is dilute, under molar concentrations of 90%, the solution acts as a strong acid and does not show dehydration power. On the other hand, when it is concentrated, above 90%, its acidic feature is omitted and its dehydration power increases. Due to its importance in chemical process and in view of all works present in the literature, one does not consider both dissociation reactions in the liquid phase and the complex formation reaction in the vapor as one, a thermodynamic model has been developed using Prigogine and Defay Chemical Theory. This model takes into account phase and chemical equilibria, simultaneously, with satisfactory results to calculate vapor-liquid equilibria of H2O-H2SO4 solutions in the whole concentration range and temperatures from 0 to 150 degrees C. (c) 2006 Published by Elsevier Ltd.