Various contributions to the thermal expansion of a liquid foam are discussed, which are related to the transfer of molecules between the liquid and the gas as the temperature changes. Experimental measurements of the thermal expansion of two types of aqueous foams are presented which can be interpreted in terms of the change with temperature of the equilibrium water vapor pressure. Because of this effect the foams expand considerably more (by a factor of similar to 1.4 under normal conditions) than what would be predicted for a closed ideal gas.