A new isothermal flow calorimeter for gases at high temperatures and pressures is described. The calorimeter employs a throttling-capillary tube to provide both the pressure drop and the heating element. Considerable care was taken to operate the calorimeter in an isothermal environment by use of a fluidized-bed thermostat. The instrument has been used to make measurements of the molar isothermal Joule-Thomson coefficient of N-2 at temperatures in the range 295 K to 473 K and at pressures in the range of 1.0 MPa to 10.0 MPa and also of CO2 at temperatures in the range 300 K to 500 K and at pressures in the range 3.0 MPa to 5.0 MPa. The molar isothermal Joule-Thomson coefficient for NI has been measured to +/-1 J . MPa(-1). mol(-1) and that for CO2 to +/- 4 J . MPa(-1). mol(-1). Agreement is found with other available experimental results and with the predictions made by the IUPAC-approved equations of state.