The nature and distribution of the components, and evolution of the suite of compounds, secreted by the sternal gland of marsupials were studied. Individuals from nine families (18 species) of marsupials and from the echidna were sampled over an 18-month period. The assay system used gas chromatography/mass spectrometry and utilised thermal desorption with cryofocusing. Parsimony analysis, constrained by an assumed phylogeny, was used to examine the evolution of the suite of compounds detected. Large interspecies variation in secretion composition was observed with acetic acid, short-chain (C₆–C₁₀) aldehydes, long-chain (>C₁₅) hydrocarbons and 1,1-bis-(p-tolyl)-ethane being constituents of the secretion of most species. The suite of compounds, however, varied from three compounds in the yellow-bellied and feathertail gliders to 41 in the koala. The most complex suites of compounds were found in the brown antechinus, red kangaroo, tammar wallaby and koala. Radical differences were observed between the secretions of related species (for example, brown antechinus and mulgara, tammar and parma wallabies, wombat and koala). Compounds appeared and disappeared repeatedly across the phylogeny. No compound constituted a synapomorphy for the Australian marsupials and only one compound was considered a synapomorphy for the Diprotodontia.