In the Spheripol process (Basell), high impact polypropylene (hiPP) is produced in two stages in series. First, isotactic polypropylene (i-PP) particles are produced in liquid propylene. These particles are transferred to a gas phase fluidized bed reactor where the elastomeric phase is produced within the isotactic polypropylene. The particulate product obtained in the commercial process is heterogeneous. This heterogeneity may be deleterious for the product performance. In this work, the origins of product heterogeneity were studied combining a detailed characterization of the product sampled from the exit lines of the homopolymerization stage (i-PP particles) and the fluidized bed reactor ( hiPP particles) of a commercial unit with a mathematical model of the process. It was found that the experimental results were consistent with equally accessible active sites of uniform activity, the residence time distribution of the catalyst in the different reactors playing the major role in product heterogeneity.