Biomass gasification in fluidized beds generates tar that can be effectively eliminated with catalysts located downstream from the biomass gasifier. Some previously obtained results from such catalytic tar elimination were hard to be understood because the tar was considered as only one or two lumps. For this reason, in this work, tar is considered in two ways: (i) as being composed of six different lumps and (ii) as a continuous mixture. For these studies, tar was sampled before and after two catalytic beds located in a slip-flow downstream from a fluidized-bed gasifier (pilot scale). Such tar samples were then completely characterized, and the tar composition was thus determined before and after the catalytic bed. The conversion of each of the species present in tar, and thus of the lumps, was calculated at different temperatures and space-times in the catalytic reactor. The two most advanced reaction networks and their corresponding kinetic models are presented here. The first one provides the evolution with gas residence time of the mean molecular weight of the tar and of the variance of the tar molecular weight distribution. The second kinetic model is based on a set of six kinetic equations and eleven kinetic constants, which are calculated here. This second model allows the species present in tar to be ranked according to their reactivity or resistance to being destroyed. Predictions from these models are consistent with all existing data on this matter.