The leaf essential oils of Zanthoxylum armatum DC (Rutaceae) from Kumaon, India, extracted by hydrodistillation, were analyzed by capillary gas chromatography (GC-FID) and gas chromatography-mass spectrometry (GC-MS). The major classes of compounds found in the leaf oils were acyclic and menthane monoterpenoids as well as simple alcohols, aldehydes and ketones. The high proportion of non-terpenic acyclic ketones, notably 2-undecanone and 2-tridecanone, and the low abundance of undec-10-en-1-al and p-phellandren-8-ol make the composition entirely new. Other constituents present in significant amounts were oxygenated monoterpenes, which include 1,8-cineole, linalool, terpinen-4-ol, and alpha-terpineol, and sesquiterpene hydrocarbons represented mainly by trans-caryophyllene, a-humulene and germacrene D. On the contrary, the oil distilled from the leaves on the second day of distillation was characterized by a high content of 2-tridecanone (27.1%) and trans-caryophyllene (7.4%), as compared with 3.5% and 4.6%, respectively, for the fresh leaves; a slight decrease in pH of the distillate was also significant. Moreover, the presence of a high 2-undecanone content followed by 2-tridecanone is being reported for the first time for Z. armatum from this region. In terms of molecular diversity, the simple acyclic ketones dominate the essential oils as compared with linalool that was reported in several previous studies on Z. armatum. Therefore, the two acyclic ketones may be utilized to establish the origin and authenticity of the material.