Heat capacities of diethyl ether in the crystalline, liquid, and vapour states in the temperature range 15–450 K, heats of fusion and vaporization, and triple points of stable and metastable crystals have been measured. Entropies of the liquid at 298·15 K and of the ideal gas in the standard state were found to be 253·5 and 342·2 J K–1 mol–1 respectively. Equilibrium constants for the reaction 2EtOH(g) Et·O·Et(g)+ H2O(g) have been calculated from thermal data for comparison with measured values.