New experimental results for the enthalpy change upon diluting aqueous solutions of sulfur dioxide as well as aqueous solutions of sulfur dioxide and ammonia in pure water at about 313 and 352 K are presented. The experimental results are used to modify a previously developed thermodynamic model for the vapor-liquid equilibrium of the chemical reacting system (SO2 + NH3 + H2O). That model is extended allowing on the one side for the formation of pyrosulfite, and on the other side for the description of experimental results for the enthalpy change upon dilution (in pure water) of (NH3 + H2O) (taken from the literature), as well as (SO2 + H2O) and (SO2 + NH3 + H2O) (from the present work). © 2004 Elsevier B.V. All rights reserved.