When polypropylene is heated in an inert atmosphere and the volatile products are condensed, the molecular weight distribution (MWD) of the product obtained is very wide. In addition, this product is very waxy, has poor flow properties, and is unusable as a liquid fuel. To improve the product quality, ZSM-5 and other catalysts are usually used in the recycling process. However, these catalysts are costly. On the other hand, if the higher boiling fractions of the volatiles evolved due to degradation are not allowed to escape, then the MWD of the final product can be improved. This paper compares the effects of degradation with and without a reflux on the MWD of the product. Initial MWD of the polypropylene determined by GPC was used for all the simulations. In solving the governing population balance equation for the polymer degradation process, the rate constant for degradation has been fitted to the total weight loss of the polypropylene at a given temperature. The problems arising from the use of moment method with an assumed gamma-distribution are also discussed. A fixed-pivot discretization technique has been adopted for solving the governing equations. (C) 2004 Elsevier Ltd. All rights reserved.