The thermal decomposition of the Cq, C,, and CS polymethylene sulfones has been investigated at 275Β°C. under redured pressure. The gaseous deromposition products, collected and identified by infrared and mass-spectrometer analysis, were found to consist primarily of sulfur dioxide, olefins, alkanes, water, and hydrogen. A mechanism is proposed in which the rate-determining step consists of a roncerted attack of the sulfone group on the @-hydrogen atom with elimination resulting in a-olefins and a sulfinic arid. The sulfinir acid rapidly decomposes to give sulfur dioxide, hydrogen, and alkyl free radicals. The final products are formed by transfer and termination reactions of the free radicals and isomerization of the olefins in the presence of sulfur dioxide. Experimental evidence confirming this mechanism consists of rate studies of polymer decomposition whose initiation was found to be independent of polymer purity and of added free-radical initiators. A further proof of the rate-determining step was obtained by comparison of the decomposition products of bis-l,4-(butanesuIfonyl)butane with those of structurally comparable sulfinic acids: 1,4-butanedisulfinic acid and 1-butanesulfinic acid.