In this paper, a similarity relationship is derived for enclosures filled with a saturated gasvapour mixture. Interest in the process arises from the need to cool electronic equipment in sealed enclosures. Firstly, an argument is presented showing that the mixture will be saturated or nearly so when vapour is generated at the highest temperature surface. Then the equations derived from the conservation of mass, energy and momentum are manipulated to a form similar to those for a single component fluid, and the approximations required are discussed. Finally, an example is given showing how the net heat transfer rate in a cavity is enhanced using this process.