Mass transfer limitation is often the controlling mechanism of the conversion rate in trickle bed reactors, where catalysts with a huge activity are employed. Modelling and interpretation of reactor performance would then require the knowledge of the proper mass transfer coefficients. Anyway, mass transfer coefficients that can be found in the literature are widespread and are obtained mostly for non-foaming and non-reacting systems. In this paper, we examined the catalytic hydrogenation of a-methyl styrene to cumene (foaming and reacting system) in an isothermal laboratory reactor and in a pilot scale adiabatic one. The first aim of the work is to propose a scale-up criterion, as a useful tool to estimate performances of a pilot scale plant from those of a laboratory scale. Moreover, a model that considers the reaction as instantaneous is proposed; the aim of the model is to demonstrate how the systems where organic liquids are involved are improperly described with the mass transfer coefficients which can be found in the literature. (C) 2004 Elsevier Ltd. All rights reserved.