The invention concerns a novel method for preparing a benzofuran or benzothiophene compound by cyclizing an aromatic compound bearing a side-chain comprising at least two carbon atoms, one of the carbon atoms being bound to the benzene cycle by an oxygen or sulphur atom, the other carbon atom is in carboxylic form and a formyl radical in ortho position relative to said chain. The inventive method is characterised in that it consists in cyclizing the latter in the presence of an efficient amount of a carbonate base in a medium comprising a carboxylic acid anhydride and optionally an organic solvent.