The application of the principle of congruence to multicomponent mixtures was tested in a novel way by mixing a symmetrical ether of side chain carbon number m with an equimolar mixture of two symmetrical ethers such that their average side chain carbon number was m. The equimolar mixture was known as a pseudo-ether of carbon number m. Excess molar enthalpies and volumes were measured using these mixtures at atmospheric pressure and at 298.15 K and the results used to test the principle of congruence. The process was repeated for mixtures of ketones and pseudo-ketones.