The single events theory is based on a detailed and exhaustive network of reactions and species. This network is generated by computer and based on some assumptions that reduce the number of parameters involved in the description of the mechanism. The single events theory has been proved to be efficient in the modelling of acid catalysed petrochemical processes. The lumping approach was intensively developed for heavy feed process where species were at thermodynamical equilibrium. The main aim of this lumping approach was the reduction of the kinetics and computational burden. The same concept can however be generalised to. any single events model, by computing molecular or lumped reactivities. Advantages of the molecular. or lumped reactivities are check on the thermodynamical coherence, comparisons with apparent kinetics, links with traditional point of view on kinetic modelling, optimisation of the computational algorithms, etc. Application to Butane isomerization modelling will illustrate this methodology. (C) 2003 Elsevier Ltd. All rights reserved.