This paper presents a mathematical relationship between the parameters of Levenspiel's Deactivation Kinetic Model (LDKM) and those of the Deactivation Models with Residual Activity (DMRA) and their evolution over time. This correlation provides an explanation for the erroneous variation obtained in the kinetic parameters (deactivation order and deactivation function) over time when LDKM is used to fit deactivation data having a certain level of residual activity, which frequently leads to systematic errors in estimating intrinsic parameters, such as activation energies. The variations of the LDKM parameters cannot in fact be related to a physical phenomenon, but are only a consequence of a mathematical artifact. The methodology developed in this work provides a valuable tool for the comparison and discrimination between different models used in kinetic studies. The equations here presented are applied to analyze the deactivation by fouling of Pt/Al2O3 reforming catalysts during methyl cyclohexane dehydrogenation. (C) 2003 Elsevier Science B.V. All rights reserved.