The isomerization of xylenes is a catalytic transformation process of metaxylene and ethylbenzene, which aims to restore the balance between both isomers and convert the ethylbenzene to xylenes. The knowledge of the catalytic deactivation behavior between stages of operation or campaigns (catalyst that has undergone successive regenerations and activations) in xylenes isomerization is very important to determine the useful life of the catalyst and, consequently, to improve the estimation of the economic viability to operate the unit, whether with a new or an old kind of catalyst. This work is a preliminary case study of catalyst deactivation, focused on pre-existing industrial data production of xylenes from a unit found in a famous industry known as Braskem, which is located in Brazil. This article involves monitoring a catalyst load throughout its useful life, which has gone through seven consecutive campaigns. The first stage of the work involved the definition of the variables that most influenced the ethylbenzene conversion and the proposition of two mathematical models, one linear and another quadratic, whose parameters were found from real plant data. The quadratic model best represented the actual behavior of the system, and the methodology used led to the conclusion that the analysis of the real ethylbenzene conversion data between the campaigns would not be sufficient to elucidate the catalyst deactivation profile due to the variability of operating conditions between the different campaigns. The catalyst irreversible activity losses (deactivation rates) between the campaigns were calculated from the quadratic models, to determine the variation of the conversion between the campaigns. Finally, an economic evaluation was done with the aim of evaluating the optimal catalyst lifetime.