The rearrangements of azulenes to naphthalenes, which occur at 350–450°, are catalysed by methyl radicals and it is suggested that skeletal rearrangement takes place in radical adducts (e.g. RC10H8· from azulene itself). Detailed analysis is reported of the products of thermolyses of the five methylazulenes, of seven dimethylazulenes, of benz-[a]- and -[f]-azulenes, and of dimethyl azulene-4,5-dicarboxylate and the derived anhydride and N-methyl-imide. The majority of the products can be accounted for by proposing that rearrangement results from (i) attack of a radical (R·) on a carbon in the seven-membered ring, (ii) migration of the attacked carbon and its substituents (e.g. CHR or CRMe) into the five-membered ring, and (iii) loss of R, H, or Me to form the naphthalene products. However the final disposition of carbons and substituents in the former five-membered ring is not understood (2-methylazulene unexpectedly produces 1- as well as 2-methylnaphthalene).