The molar heat capacity at constant pressure C-p,C-m and the differential Joule-Thomson coefficient mu of 1,1,1,2-tetrafluoroethane, 1,1-difluoroethane, and of a mixture of equal amounts of both substances have been measured by flow calorimetry. The investigation covered gaseous and supercritical states from the temperatures 298 K to 423 K and pressures up to 2.5 MPa, and the results were consistent with accurate (p, V-m, T) measurements from the literature. The excess values of C-p,C-m were positive in the investigated range of state, and the excess values of the isothermal throttling coefficient Phi(m) = -mu . C-p,C-m were negative. This is a consequence of the fact that the intermolecular attraction is stronger in the mixture than in the pure substances, whereas the opposite mixing behaviour is usually observed in gaseous mixtures in that range of state. This is shown explicitly by comparison with a mixture of equal amounts of substance of trifluoromethane and sulphur hexafluoride, which had been investigated previously by the same measuring technique.