An isenthalpic process or isoenthalpic process is a process that proceeds without any change in enthalpy, H; or specific enthalpy, h.

If a steady-state, steady-flow process is analysed using a control volume, everything outside the control volume is considered to be the surroundings.

Such a process will be isenthalpic if there is no transfer of heat to or from the surroundings, no work done on or by the surroundings, and no change in the kinetic energy of the fluid. This is a sufficient but not necessary condition for isoenthalpy. The necessary condition for a process to be isoenthalpic is that the sum of each of the terms of the energy balance other than enthalpy (work, heat, changes in kinetic energy, etc.) cancel each other, so that the enthalpy remains unchanged. For a process in which magnetic and electric effects (among others) give negligible contributions, the associated energy balance can be written as

${\displaystyle dK+du=Q+W}$

${\displaystyle du=d(h-PV)=dh-d(PV)}$

${\displaystyle dK+dh-d(PV)=Q+W}$

If ${\displaystyle dh=0}$ then it must be that

${\displaystyle dK-d(PV)=Q+W}$