The N2O4 isomerization in gas phase has an energy barrier of 31 kcal mol(-1) at 298 K. This energy barrier may be reduced due to the interaction of the N2O4 isomers with water or nitric acid clusters adsorbed on surfaces. The Gibbs free energy barrier for this reaction in water medium is estimated to be reduced to 21.1 kcal mol(-1) by using the ab initio calculations and the polarizable continuum model (PCM). By using the transition state theory (TST), this model estimates that the N2O4 isomerization may be as fast as 2.0 x 10(-3) s(-1) in aqueous phase at room temperature, which confirms the Finlayson-Pitts model for the heterogeneous hydrolysis of NO2 on surfaces. The activation energy of the N2O4 isomerization is about 21 kcal mol(-1). The rate coefficient for this reaction is considerably fast, 1.2 x 10(-2) s(-1), in aqueous phase at T = 373 K.