Spearmint oil (essential oil of Mentha spicata) was extracted from Turkish mint-plant leaves using supercritical CO2 in a basket-type extractor. Effects of temperature, pressure, extraction time, amount of CO2, and entrainer (ethanol) concentration on composition of essential-oil extracts and oil yield, relative to hydro-distillation, were investigated. Relative yields were as high as 80%. Monoterpenes fraction in extracts was inversely correlated with relative oil yields. Monoterpenes were preferentially extracted by CO2 due to their high vapor pressure, low molar mass and low polarity. Supercritical extraction presented advantages over the conventional hydro-distillation by yielding extracts lower in monoterpenes fraction and by enabling high yields at temperatures safe for the heat-sensitive essential oil.