Glycerolpropylsilane bonded phases have been found to adsorb peptides and proteins via ionic interactions. In this paper high-performance liquid chromatography separation of glucagon and related compounds, using a Diol-silica matrix, is described. Crystalline, commercial preparations of glucagon, when analyzed on LiChrosorb Diol columns eluted with low-ionic-strength acidic buffers, contained up to four contaminant peaks, in different numbers and ratios. Three of these contaminants, called A, C, and D, were recovered and characterized. Contaminant A, representing a few percent of the total, was a mixture of mono- and didesamidoglucagon, as shown by treatment with bis(I,I-trifluoroacetoxy)iodobenzene, with which it is possible to differentiate between carboxamide and carboxylic acid residues. Contaminant C, ranging from 0 to about 30% of the total, was N-terminal degraded glucagon. Contaminant D, ranging from a few percent to about 25% of the total, was (Met27 sulfoxide) glucagon.