Three essential oils, tea tree oil from Melaleuca alternifolia and two lavender oils (Lavandula angustifolia and Lavandula intermedia lavandin) produced by steam distillation, have been examined by gas chromatography-mass spectrometry (GC-MS) and by size exclusion chromatography (SEC) using 1-methyl-2-pyrrolidinone (NMP) as the eluent. This mode of operation of SEC normally produces bimodal chromatograms for petroleum residues, asphaltenes, and coal-derived liquids. Because these volatile oils consist of relatively small molecules, they were seen as a test of the SEC mechanism in eluting terpene molecules and their oxygenated derivatives as small molecules rather than through the formation of aggregates that may masquerade as large molecules. GC-MS confirmed the small molecular types, and the oils eluted from the SEC column only as small molecules. The inclusion of oxygen favored early elution from SEC but not into the excluded region.