The gas phase polymerization of butadiene with a heterogeneous Ziegler catalyst based on neodymium was studied by using videomicroscopy and a stirred tank reactor of laboratory scale. By videomicroscopy the start up of polymerization and the particle growth of individual catalyst particles was measured at different reaction conditions. In the stirred tank reactor, the kinetics and the particle size distribution of the polymer formed was studied. For modeling of the reactor performance, a single particle model based on the uniform site model was implemented in a reactor environment model. A dynamic population balance is used to simulate the change of the particle size distribution during the polymerization. The population balance is solved simultaneously with the single particle model to include the influence of diffusion limitation on the polymerization. The experimental results and the simulation show a good agreement.