Measurements of kinetic data in experimental catalytic reactors are often plagued by non-constant catalytic activities, for example, due to unexpected fouling of catalysts, even during a relatively short time interval. The purpose of the present work is to illustrate how the data analysis in the composition space can be conveniently employed to derive appropriate kinetic information from such ill-defined rate data. The catalytic reaction considered is the hydrogenation of propene, butene, and their mixtures at 3.5MPa over a presulfided Co-Mo catalyst supported on gamma-alumina, in which volatile polymer formation of butene at low reaction temperatures and depolymerization at higher temperatures are observed to occur. The present work reports the data analysis methods employed, which eventually have led to consistent reaction pathways and to the determination of relative rate constants associated with them. The results reported herein may be useful for designing and operating commercial hydrotreaters where various C₃/C₄ olefin mixtures are hydrogenated. The methodology adopted here may be extended to any complex reaction system such as catalytic reforming of naphtha.