A requisite for successful biochemical applications of mass spectrometric sequencing of peptides is the development of a suitable micromethod for transforming the peptide into a volatile derivative. We have now developed a new mass spectrometric sequencing procedure which offers advantages in speed of analysis and convenience over previously described methods. The new procedure depends upon the conversion of an Nprotected peptide trimethyI phenyl ammonium salt into a volatile peptide methyl ester using the solid probe heater of a mass spectrometer as a reactor. Under these conditions the residues of Tyr, Asp and Glu are completely methylated In the side chain, Trp and His are partially methylated whilst Gin and Asn peptides are stable under the derivatisation conditions. Apart from these desirable side reactions, the mass spectra of the volatile derivatives obtained by the new pyrolytic esterification procedure are essentially identical to those obtained by conventional methods.