AbstractComparison of the acetyl‐, trifluoroacetyl‐, benzyloxycarbonyl‐, methoxycarbonyl‐, ethoxycarbonyl, methylaminocarbonyl, phenylaminocarbonyl‐, phthanoyl‐, and stearoyl‐derivatives of glycylleucylphenylalanine methyl ester has shown that acetyl derivative is the most volatile. The relative abundances of the sequence ions in the mass spectra od these compounds have been determined and compared: the acetyl derivative provides the best combination of volatility and abundance of sequence ions. Evidence is presented to show that the use of acid‐catalysed esterification procedure for the chemical modification of peptides prior to mas spectrometric investigation can lead to degra