Samples of Eucalyptus globulus Labill. subsp. globulus essential oil obtained from trees of different ages, growing in maritime and inland Portuguese regions, collected in May and November periods (1991-1992), were submitted to quantitative analysis. The results obtained were interpreted using principal component analysis and cluster analysis, considering the 25 abundantly found compounds. These compounds were grouped according to their variation patterns, leading to the definition of five groups, four of which have shown to be significant in the differentiation of young and adult leaves. Young leaves, when compared to adult leaves, were found to have higher oil yields, with lower percentage of group II (in which 1,8-cineole is included) and III compounds and higher percentages of compounds from groups I and IV. Variations in the oil composition due to season and geographic localisation were found not to be significant.