An isochoric process, also called an isometric process, is a thermodynamic process in which the volume stays constant; . This implies that the process does no pressure-volume work, since such work is defined by
where P is pressure (no minus sign; this is work done by the system).

By applying the first law of thermodynamics, we can deduce that

for an isochoric process: all the heat being transferred to the system is added to the system's internal energy. If the quantity of gas stays constant, then this increase in energy is proportional to an increase in temperature,
where CV is molar specific heat for constant volume.

On a P-V diagram, an isochoric process appears as a straight vertical line.

See also: isobaric process, isothermal process, adiabatic process, cyclic process.