Major problems in the use of the fugacity model can derive from the lack of availability and the poor quality of basic data on the physico-chemical properties of the molecules. Theoretically, the only data essential to apply the fugacity model are water solubility and vapour pressure. Even for these elemental properties, much uncertainty exists. In the most frequently consulted reference books, such as The Pesticide Manual (Worthing and Walker, 1987), data are often expressed in vague terms or are not available at all (e.g., solubility in water: `negligible'; vapour pressure: `less than . . .'). A recent review (Suntio et al., 1988) of Henry's law constants for pesticides demonstrates that data in the literature are often conflicting and that the selection of the most reliable data is a very vexing task.อ