Abstract The BF 3· Et 2 O-and the CH 3 SO 3 H-catalysed rearrangements of 10 α- vinylcyclobutanones have been examined. With little acid, the β, β-dialkyl derivatives 1 were transformed into linear dienones 2 and 3; with more acid, they were converted into cyclopentenones 4 by Nazarov cyclisation of initially formed 2/3. The β-monoalkyl (including the β, γ-dialkyl) derivatives 7 rearranged only with a high acid concentration to afford the ...