A thermodynamic process (also called a throttling process) in which enthalpy is conserved,

The locus of maxima for T versus p curves (where the Joule-Thomson coefficient ) is called the inversion curve for the process. The inversion curve exhibits a maximum inversion temperature. If the final pressure is smaller than the initial pressure, then T decreases for .

To cool via the Joule-Thomson process, a substance must first be pre-cooled to below the maximum inversion temperature.