A new methodology is presented to measure water-air partition coefficients (Henry's constants) of volatiles, using APCI-MS. Significant advantages over other Henry's constant determination methods include the short measurement and sample preparation time and the possibility for simultaneous measurement of multiple volatiles. The methodology is validated by obtaining good agreement with reliable literature values for a series of 2-ketones. The methodology is further explored for eight key volatiles typically found in citrus fruits, including the temperature dependence of the Henry's constant. Using these data can improve estimates of flavor losses during processing and volatile release during consumption.