A very simple and easy method for measuring water-hexadecane partition coefficients (Pw,16) of a wide range of solutes (log Pw,16 ranging from -2.7 to 4.4) by headspace gas chromatography has been developed. This method requires no standard solution of the solute and no calibration of the detector. It only requires that the solute be volatile enough to be measurable in the headspace of the water-hexadecane two-phase system and the detector be linear. The precision of measurement is often better than 2% in log Pw,16· From the measured water-hexadecane partition coefficient and gas-hexadecane partition coefficient (log K16), a gas-water partition coefficient and hence the limiting activity coefficient (γ∞) in water can be calculated. The γ∞ values obtained this way compare very favorably (ca. 5%) to those directly measured by other techniques such as direct headspace gas chromatography and inert gas stripping. This method should be equally applicable to the measurement of partition coefficients between other immiscible solvent systems.