This paper deals with some current problems of single-particle modelling of fastly growing porous polypropylene particles. Precise kinetic measurements, investigations in monomer absorption, and microscopic morphology studies have been performed to prove the model assumptions usually made by several investigators. Both liquid pool and gas-phase polymerization have been carried out in stirred batch reactors and rates have been measured by calorimetric and mass flow measurements. Silica-supported single-site metallocenes and MgCl2-supported Ziegler-Natta catalysts have been studied for comparison. Kinetics, absorption measurements and SEM photographs of 'normal' and microtomed catalysts, pre-polymers and polymer particles emphasize the need to develop more realistic single-particle models that respect the real powder morphology. Polymeric flow models, core-shell models and multi-grain models as currently used are not adequate.