The Joule-Thomson effect, also known as the Joule-Kelvin effect, is the change of temperature that occurs when a gas expands through a porous plug. The change of temperature is proportional to the pressure difference across the plug. The Joule-Thomson coefficient is the change of temperature per unit change of pressure.

The temperature change is due to a departure of the gas from Joule's laws, the gas performing internal work in overcoming the mutual attractions of the molecules and thus cooling itself; and partly to deviation of the gas from Boyle's law. The latter effect can give rise either to cooling or heating, depending upon the initial temperature and pressure difference used. For a given mean pressure, the temperature at which the two effects balance, resulting in no alteration of temperature, is called the inversion temperature. Gases expanding through a porous plug below their inversion temperature are cooled, otherwise they are heated. See also cryogenics.