Water solubilities, activity coefficients, and vapor pressures of some terpenes and terpenoids were determined from two sets of experiments. The first involved liquid-liquid contacting until saturation of an aqueous layer, which gave solubility and activity coefficient at infinite dilution gamma(infinity). The second set involved stripping a solute from a nonsaturated aqueous solution by an air stream. This last experiment gave the product gamma(infinity)P degrees from which vapor pressure P degrees was deduced. Reliability of the results was established from predicted P degrees values, and data for 12 compounds are now available. Data obtained at 25 degrees C showed low solubilities (0.037-0.22 mmol/L) and high activity coefficients (10(5)-10(6)) for terpenes, whereas oxygenated monoterpenes exhibited solubilities 20 orders of magnitude higher, in the range of 2-20 mmol/L and 10(3)-10(4) for activity coefficients. Vapor pressures ranged from 100 to 550 Pa for terpenes and from 1 to 130 Pa for terpenoids. Attempts to estimate water solubility showed that recent equations in the Literature are unable to predict this property with an accuracy better than 140%.