Low-and high-density polyethylene and polypropylene have been degraded in a fixed-bed now reactor system with and without H-gallosilicate catalyst at 375-550 degreesC to investigate the product distribution and the catalyst stability. The thermal degradation of the polyolefins mainly produced waxy hydrocarbons, with the yield largely depending on the polymer type. On the other hand, the catalytic degradation over the gallosilicate yielded lighter hydrocarbon mixtures that were rich in valuable aromatic components, mostly benzene, toluene, and xylenes. The product distribution was influenced little by the structure of the polymers to be degraded. This can be explained by a mechanism involving frequent skeletal isomerization of the decomposed fragments. The unsaturated fragments, which were the most abundant and thereby the most important reaction intermediates, rapidly isomerized on the acidic gallosilicate, and the resulting isomers were distributed in thermodynamically equilibrated concentrations. The catalytic degradation of polyolefins thus proceeds through similar intermediates regardless of the structure of the degrading polymers, leading to almost the same product distributions. The gallosilicate exhibited a stable catalytic activity for the degradation of polyolefins when reused, because of a very low yield of coke deposited on the catalyst surface.