The invention concerns a method for preparing a carboxylic acid comprising at least 3 carbon atoms. The invention concerns the preparation of such a carboxylic acid by a method using formic acid, as hydrocarboxylation reagent. The method for preparing a carboxylic acid comprising at least 3 carbon atoms, from a formic acid source is characterised in that it consists in reacting between 100 DEG C and 200 DEG C an unsaturated organic compound comprising at least 2 carbon atoms and formic acid, in the presence of an efficient amount of an iridium-based catalyst associated with an iodated promoter.
