Unproportional pressure variations are observed in a closed thermosyphon during the transition from subcritical to supercritical states and vice versa. Such a stepwise pressure variation is encountered at lower pressures and with a larger step when the heat flow rate is large and the tube is steep. The unsteady behaviour is explained by an instability occurring in the two phase counterflow of vapour and liquid : at high subcritical pressures, the densities of liquid and vapour become equal and reduce buoyancy and separation of streams. This brings forth the collapse of the internal circulation flow. Only when this flow is re-established in the one phase supercritical region—with a preceding unproportional pressure increase—steady-state operating conditions are obtained again.