The Diels-Alder adduct of 3-vinylbenzo[b]thiophen and 1,4- naphthaquinone has been shown to undergo a novel base-catalysed rearrangement, in which its partly hydrogenated 12-thianaphtho[2,3- a]fluorene structure is replaced by a 5-thianaphtho[2,3-b]fluorene structure.