A lumped kinetic model for the liquid-phase oxidation of p-xylene to terephthalic acid catalyzed by cobalt naphtenate is presented. It is assumed that all the reactions are zeroth- and first-order with respect to gaseous and liquid reactants, respectively. The reliability of the developed model is investigated by comparison with experimental data obtained in an isothermal semi-batch oxidation reactor where the gas and the liquid phase are well mixed. The experiments included different values of the initial concentration of liquid reactants, two gaseous reactants (i.e. pure oxygen and air), various levels of catalyst concentration (from 1.67 to 33.3x10(-4) mol/kg(l)), and temperature values in the range 100-130 degrees C. A semi-batch gas-liquid reactor model which accounts for the complex nature of the involved reaction network, as well as for inter- and intra-phase mass transport processes of both reactants and products is presented. The model, whose reliability is tested by comparison with suitable experimental data obtained in the semi-batch oxidation reactor, accounts also for the interaction between the involved chemical reactions and the precipitation kinetics of both 4-carboxybenzaldehyde and terephthalic acid. It is shown that the model describes the reactor behavior in any of the regimes which may prevail depending upon the operating conditions.