The genus Mentha L. (Lamiaceae) is represented in the world by 62 taxa including 18 species and 11 hybrids. In Turkey, 15 taxa exist. In the course of the present work, 90 samples belonging to 11 Mentha taxa collected from the Marmara region of Turkey in north western Turkey were hydrodistilled to obtain essential oils, which were then subjected to gas chromatographic (GC) and GC/mass spectrometric (GC/MS) analyses. Fifty-six percent of the samples contained 3-oxo-1,2-epoxy type compounds (piperitenone oxide, cis- and trans-piperitone oxide) as the main constituents in their oils, whereas 25% contained 3-oxo compounds (menthone, isomenthone, pulegone, menthofuran, piperitone, piperitenone and isopulegol). Ten percent of the samples comprised 2-oxo compounds such as carvone, cis-and trans-dihydrocarvone; 2% contained 1,8-cineole and 1% contained trans-sabinene hydrate as the main constituent in the oils; 5% belonged to an unknown category found in the five oils of Mentha longifolia.