Detailed GC and GC/MS analyses of essential-oil samples obtained by hydrodistillation of Morus alba L. and M. nigra L. leaves (four samples) allowed the identification of 131 constituents representing 95.1 – 96.4% of the total GC peak areas. The most abundant compounds classes were alkanes, diterpenoids, carotenoid-derived compounds and fatty acid-related constituents with trans-phytol (7.9 – 71.2%), (E,E)-geranyl linalool (0.2 – 8.0%), (Z)-bovolide (8.1%) and n-chain alkanes (in total, 17.5 – 52.4%) as the major constituents of the analyzed samples. In general, there were only quantitative differences noted between M. nigra essential oils from fresh and dry leaves. The most discernable changes included a variation in the content of the major constituents (e.g. the relative amount of trans-phytol and the total amount of alkanes decreased by 5.8% and ≈ 2%, respectively, while that of (E,E)-geranyl linalool increased by 7.8%). On the other hand, the composition of M. alba essential oil was much more significantly affected by the drying process. The highest quantitative differences were noted for trans-phytol, geranyl acetone, and all isomers of 4,6,8-megastigmatrien-3-one. Also, a rare plant metabolite, (Z)-bovolide, characteristic of leaf senescence, was only identified (8.1%) in the essential oil of M. alba dried leaves.