Gas-phase polymerizations have been executed at different temperatures, pressures, and hydrogen concentrations using Me2Si[Ind](2)ZrCl2 / methylaluminoxane / SiO2(Pennsylvania Quarts) as a catalyst. The reaction rate curves have been described by a kinetic model, which takes into account the initially increasing polymerization rate. The monomer concentration in the polymer has been calculated with the Flory-Huggins equation. The kinetic parameters have been determined by fitting the reaction rate curves with the model. At high temperatures, pressures, and hydrogen concentrations a runaway on particle scale may occur leading to reduced polymer yields. The molecular weight and molecular weight distribution of the polymer samples could be described by a "two-site model." At constant temperature the chain-transfer probability of sites 1 and 2 depends only on the hydrogen concentration divided by the monomer concentration. (C) 2001 John Wiley & Sons, Inc.