A range of inorganic and organoelement halides was evaluated as acidic promoters of direct Nin-formylation of tryptophan. In addition to Me3SiBr, the less expensive PBr3 was found to be highly efficient and was selected for further optimization. A convenient and reproducible synthetic procedure for Nin-formyltryptophan hydrobromide developed in this way was scaled to 150 mmol and successfully extended to some derivatives of Trp and closely related indoles as detailed in the present paper. The scope of the method seems to be restricted to indoles substituted at C-3. Copyright © 2014 European Peptide Society and John Wiley & Sons, Ltd.