(NIn)-Formyl protective group of tryptophan has been introduced as a base/nucleophile-labile protective group. It has long been known that a free Nα-amino group of the peptide can serve as a nucleophile: an irreversible formyl NIn → NH2 transfer is consistently observed when deformylation is performed last on an otherwise deprotected peptide that possesses free Nα-amino group. Obviously, this particular side reaction should be expected any time free amino group is exposed to Trp(For), but, at the best of our knowledge, has never been reported in the course of Boc-SPPS. In the present communication, we describe a set of appropriately designed model experiments that permitted to detect the title side reaction both in solution and in solid-phase reactions. We observed intermolecular formyl group transfer with a model compound, Trp(For)-NH2. Importantly, we also observed this migration on solid support with the rate roughly estimated to be up to 1% of residues per minute. We also observed that the formyl-group transfer reaction occurred in a sequence-dependent manner and was suppressed to a non-detectable level using ‘in situ neutralization’ technique. Because this side reaction is sequence dependent, there might be situations when the rate of the formation of Nα-formyl termination by-products is significant. In other cases, the Nα-For truncated by-products would not contaminate the final peptide significantly but still could be a source of microheterogeneity. Copyright © 2013 European Peptide Society and John Wiley & Sons, Ltd.