Gas-liquid and gas-liquid-solid reactors are frequently characterized by the presence of components, reagents or products, that are partitioned between liquid and vapor phase. In this case we have a difficulty in describing the kinetic behaviour of such type of reactors when it occurs that: (i) the amount of partitioned component in the two phases is never negligible in both phases: (ii) the vapor-liquid equilibrium could have a non-ideal behaviour. We need therefore, to introduce in the kinetic model the mass balance equations for describing the partition of the components affecting their concentration in liquid phase also considering, when necessary, the non-ideal behaviour of the involved phases. Vapor-liquid equilibria (VLE) in reactive systems are poorly treated in the literature, especially for reactions occurring at high pressures. In the present work we will examine the different possibilities occurring in practice and the methods to solve them. Some practical examples are reported for better explaining both the theoretical and practical approach. (C) 2003 Elsevier Science B.V. All rights reserved.