Abstract Curcuma longa leaves, an unexploited agricultural waste in the hilly terrains of the western Himalayas in India, were exploited to isolate essential oil. The efficiency of vacuum distillation (150 mmHg) over conventional distillation at atmospheric pressure was assessed on the quantitative and qualitative characteristics of these leaf oils, known for high monoterpene content and possessing lower boiling points. The oil yields were comparatively lower in vacuum distillation (0.25%) than conventional distillation (0.28%). GC and GCâMS analyses revealed that the vacuumâdistilled oil and the conventionally distilled oil contained maximum total monoterpenoids (88.2%) and sesquiterpenoids (45.7%), respectively, followed by âresidual oilâ, produced by redistilling the leaves at atmospheric pressure after vacuum distillation. Further, the ratio of monoterpenoids to sesquiterpenoids in the vacuumâdistilled oil was 10 times higher (10.5) than the conventionally distilled oil (1.0). These variations are attributed to the fact that in vacuum distillation, the constituents with lower boiling points are easily distilled off, leaving behind the higher boiling point constituents, which are otherwise extracted in the conventional distillation. In antifungal assays of these essentail oils against the fungal pathogens Fusarium oxysporum f. sp. dianthi and Alternaria dianthi , infecting carnations, and F. oxysporum f. sp. gladioli and Curvularia trifolii f. sp. gladioli infecting gladioli, the residual oil showed better antifungal activity over other oil samples at each of the tested concentrations. Either the high content of sesquiterpenoids or the combined effect of the composition/ratio of monoâ and sesquiterpenoids might have enhanced the antifungal activities. These studies clearly established the significance of a waste yielding useful products and the usefulness of the modified water distillation methods for producing essential oils with highâquality profiles in terms of chemical composition and antifungal activity, except for oil yields. Copyright Â© 2007 John Wiley &amp; Sons, Ltd.