Henry's law solubility constant  for a gas 2 in a mixture M of two solvents 1 and 3 depends on the individual constants for each solvent,  and  according  to:
Where ,  are the molar ratios of each solvent in the mixture and a is the interaction parameter of the solvents from Wohl expansion of the excess chemical potential of the ternary mixtures.
A similar relationship can be found for the volatility constant , by remembering that  and that, both being positive real numbers, , thus:
For a water-ethanol mixture, the interaction parameter a has values around  for ethanol concentrations (volume/volume) between 5% and 25%.