Genome sequencing projects produce large amounts of information that could be translated into potential protein sequences. Such amounts of material continuously increase protein database sizes. At present, 15 times more protein sequences are available in the SWISS-PROT and TrEMBL databases than 8 years ago in SWISS-PROT. One of the methods of choice for protein identification makes use of specific endoproteolytic cleavage followed by the MALDI-MS analysis of the digested product. Since 1993, when this technique was first demonstrated, the conditions required for a correct identification have changed dramatically. Whilst 4–5 peptides with an accuracy of 2–3 Da were sufficient for a correct identification in 1993, 10–13 peptides with less than 60 ppm mass error are now required for Human and E. coli proteins. This evolution is directly related to the continuous increase of protein database sizes, which causes an increase of the number of false positive matches in identification results. Utilisation of an information complement deduced from the primary protein sequence in the process of identification by peptide mass fingerprints can help to increase confidence in the identification results. In this article, we propose the exchange of labile hydrogen atoms with deuterium atoms. The exchange reaction with optimised techniques has shown an average 95% of hydrogen/deuterium exchange on tryptic peptides. This level of exchange was sufficient to single out one or more peptides from a list of potential candidate proteins due to the dependence of hydrogen/deuterium exchange on the peptide primary structure. This technique also has clear advantages in the identification of small proteins where direct protein identification is impaired by the limited number of endoproteolytic peptides. Then, primary sequence information obtained with this technique could help to identify proteins with high confidence without any expensive tandem mass spectrometer instruments.