Abstract: The present invention relates to a method of identifying a polypeptide, which method comprises the steps of (a) derivatization of the N-terminus of the polypeptide, or the N-termini of one or more peptides of the polypeptide, with at least one acidic reagent which comprises a sulfonyl moiety coupled to an activated acid moiety to provide one or more peptide derivatives; analyzing at least one such derivative using a mass spectrometric technique to provide a fragmentation pattern, and (c) interpreting the fragmentation pattern obtained, wherein the peptide or polypeptide is immobilized to a solid support at least during step(a). Furthermore, the present invention also relates to a kit for identifying a polypeptide by a mass spectrometric technique.