The influence of catalyst concentration, i.e. cobalt naphthenate, on product distribution and kinetic constants of the lumped kinetic scheme of liquid-phase p-xylene oxidation proposed in previous works (cf Cao et al., 1994a, b) is investigated. The experiments involving various levels of catalyst concentrations (from 1.67 to 33.3 x 10(-4) mol/kg,) are conducted in an isothermal semi-batch oxidation reactor where both the gas and the liquid phase are well mixed. The dependence of the kinetic constants of the lumped kinetic scheme on the catalyst concentration is examined. In addition, the interaction between the chemical reactions of the lumped kinetic scheme for p-xylene oxidation to terephthalic acid and the precipitation kinetics of both 4-carboxybenzaldehyde and terephthalic acid is analyzed theoretically. A semi-batch gas-liquid reactor model which incorporates the description of the above phenomena allows us to identify their interplay.