Abstract: The present invention relates to a bismuth-type promoter and its use as a Lewis acid catalyst for acylation reactions of aromatic compounds. The promoter corresponds to the formula MY&mgr;−q&xgr;q, where M represents a &mgr;-valent and at least trivalent element in the cationic form, preferably known to give Lewis acids, where Y is a monovalent anion or a monovalent anionic functional group, where &xgr;− represents an anion or an anionic functional group carrying a sulfonyl functional group carried by a perhalogenated atom, preferably a perfluorinated atom, more preferably a perfluoromethylene (—CF2—); and where q is an integer advantageously chosen within the closed range (comprising the limits) ranging from 1 to (&mgr;−1) (that is to say, 1 or 2 when &mgr; is 3). The present application also relates to processes for the preparation of the promoter.