The addition of a solvent to heavy crude oil causes reduction in its viscosity and facilitates extraction. Existing data on swelling of oil in the presence of solvent in vapour form have been analyzed using the Flory-Huggins theory. The model uses volume fractions and hence problems related to an appropriate average molecular weight of oil to use in the calculations are avoided. The data expressed in terms of swelling as a function of solvent partial pressure fit well with the model when the solvent vapour can condense and is miscible with oil or immiscible with oil or even when condensation is not possible. We are also able to predict when phase separation can occur (which we take to be asphaltene precipitation) in the solution although the above experiments have not looked at this aspect.