Nitration of anisole and 4-methyl anisole with aqueous nitric acid involves liquid-liquid two phase (organic phase and aqueous acid phase) mixtures of nitric acid-water-anisole/4-methyl anisole. Nitric acid is not dissociated completely into nitronium ion, which is the nitrating agent and knowledge of its concentration in both phases is very important to study nitration reaction. The e-NRTL model modified for incomplete dissociation of nitric acid for predicting concentrations of nitronium ion, undissociated nitric acid, and water and anisole/4-methyl anisole in both phases has been used. Computations of phase split, extent of ionization of nitric acid were calculated by minimizing Gibbs free energy with modified Nelder-Mead procedure. Equilibrium data of liquid-liquid binary and ternary systems were collected experimentally and interaction parameters were obtained by fitting these data. LLE data for the studied systems, ionization equilibria of nitric acid. i.e. the concentration of nitronium ion in both phases and also methods for phase equilibria, which are not reported earlier are presented. (c) 2011 Elsevier B.V. All rights reserved.