An apparatus has been designed and constructed which is capable of measuring the adiabatic Joule‐Thomson coefficient &mgr; and the isothermal Joule‐Thomson coefficient &phgr; for gases at low temperatures and high pressures. The apparatus was calibrated with nitrogen gas using pressures as high as 73.82 atm, and temperatures as low as 155.5 K. Furthermore, measurements on three methane‐ethane mixtures were carried out for a temperature range from 206.35 to 274.86 K and a pressure range from 3.37 to 42.24 atm for the adiabatic mode; and a temperature range from 222.22 to 274.70 K, and a pressure range from 3.77 to 62.93 atm for the isothermal mode.