In the Fischer-Tropsch synthesis of paraffins the use of transient operating conditions inhibits the formation of high mol. wt. compds. The mol. wt. distributions are described by a simple kinetic model which, when applied to the exptl. results, provides numerical data on the rate consts. of the propagation reaction. With two differently prepd. Ru/g-Al2O3 catalysts under transient operating conditions, the chains grow at a rate of about one CH2 group per min. The low overall activity of these catalysts is due to the very low intrinsic activity of the exposed Ru atoms and not to a low no. of active surface atoms.