The thermal degradation of polyphenylene-ether based on 2,6-dimethylphenol has been semi-quantitatively studied by experiments and simple computer simulation. The polymer was thermally aged under several different conditions prior to thermal degradation, because rearrangement during ageing was found to be one of the most important processes. Although thermal ageing increased the yield of dimeric scission products, whose origins were supposed to be methylene bridges, there was not a clear increase in monomeric products. The thermal degradation was too complicated to analyse without computer simulation. A simple degradation model was designed and the cleavage rates of the ether, the methylene bridge and the side chain were simulated in order to be able to explain the distribution of the scission products. The results were not totally conclusive, but the degradation rates gave information on the cleavage. Several figures were interesting, for example: (1) the rearranged chain was calculated to be about 60 or 80% of all chains, (2) the cleavage rate of the ether bridge was about 9 times lower than that of the methylene bridge and (3) the cleavage rate of the side chain was almost the same as that of the ether bridge.