A functional group contribution scheme based on a hydration model predicts Henry's Law constant of aqueous hydrocarbons as a function of temperature and pressure. It uses the direct link of Henry's Law constant with the Gibbs free energy of hydration, as well as its temperature and pressure derivatives accessible from calorimetric and volumetric data. A semitheoretic model inspired by the fluctuation solution theory with five adjustable parameters was used for expressing the temperature and pressure dependence of ten functional contributions. A consistent group contribution scheme for hydration properties at 298 K and 0.1 MPa was taken from literature to constrain the model at reference conditions. Parameters for hydrocarbon functional groups were adjusted by simultaneous correlation of over 1, 000 data points on Henry's Law constant and about 550 values on derivative hydration properties (enthalpy, heat capacity, and volume) available in a wide range of temperature and pressure. The resulting group additivity scheme can be used for predicting Henry's Law constant for alkanes, alkenes, alkylcycloalkanes, and alkylbenzenes up to 570 K and up to 100 MPa. Average accuracy of the predictions varies from 20 to 40% at ambient to high temperatures, respectively, and semiquantitative estimates are possible up to the critical point of water.