Soon after the beginning of the 20th century O. Dimroth and R. Zoeppritz showed that formanilide behaves as a formylating agent when in the presence of phosphorus oxychloride (POCl3). The potential for carbon-carbon bond-forming reactions of halomethyleneiminium salts in organic synthesis is by no means confined to Vilsmeier-Haack formylation of activated aromatic nuclei. The physical properties of many halomethyleneiminium halides have been described including many bromomethyleneiminium bromides and perbromides. The nature of the group or halogen atom bonded to the iminium carbon atom, and the nature of the counter anion proved to be difficult to establish. The dialkylamides of aliphatic and aromatic monocarboxylic acids have been shown to form adducts with POCl3. Phosgene, despite its toxicity, reacts with acid amides to give some of the purest samples of chloromethyleneiminium chlorides obtained by any method. Carbonyl bromide reacts with acid amides to give bromomethyleneiminium salts that are usually very labile.