Australian tea tree oil from Melaleuca alternifolia and M. linariifolia is well known for exhibiting antibacterial, antifungal, and general antiseptic properties. Hence characterization of tea tree oil is important, as this leads to a better understanding of the important components contributing to the observed properties of the oil. GC/MS is widely used for this task. However, reliable identification of oil components is not always possible using mass spectral data only. Retention times or retention indices are often used to support the MS data. The present study investigated a number of different tea tree and other Melaleuca sp. oil samples, and reports qualitative and quantitative data for each sample. Characterization of the individual oil components was performed using a private MS library, which incorporates an interactive linear retention index filter. The method described in this report increases the ease of characterization of individual oil components, and allows a greater number to be reliably assigned. More than 80% of the components in the samples studied are now reliably identified.