A gas-mixing flow calorimeter has been used to measure the excess molar enthalpy HmE of (nitrogen + chloromethane)(g), (nitrogen + chloroethane)(g), and (chloromethane + chloroethane)(g) at standard atmospheric pressure over the temperature range 250 K to 390 K. The measurements are analysed using the corresponding states correlation of Tsonopoulos with the acentric factor omega as a disposable parameter. From the value of w that fitted the HE measurements, values of the second virial coefficient B and the isothermal Joule-Thomson coefficient phi = B - T(dB/dT) were calculated. The values of B for chloromethane are in good agreement with other work, but at low temperatures those for chloroethane are less negative than previous measurements. The excess enthalpy of (chloromethane + chloroethane) is fitted using the mean value of w for the pure components.