The objective of this study was to evaluate impact of light and available oxygen on the chemical composition of three selected essential oils. Aliquots of immortelle (Helichrysum italicum), silver fir (Abies alba) and prickly juniper (Juniperus oxycedrus) essential oils were exposed to UV-A irradiation in the presence of atmospheric oxygen as well as in the presence of inert gas. The compositions of fresh and irradiated samples were studied by GC/MS. Each oil showed an individual response to the applied conditions. In immortelle oil, dominant process was phototransformation of g-curcumene to italicene, isoitalicene and ?-curcumene. Since g-curcumene is one of the major components of immortelle essential oil, exposure of this oil to light can cause significant changes in primary composition and thus quality. In silver fir and prickly juniper oil irradiation caused only slight changes among sesquiterpenes that are present as minor components. Both oils were found to be photostabile and insensitive to the presence of atmospheric oxygen.