Abstract: A vapor pressure pump for delivering a liquid into a system operating at a higher pressure or located at a higher level by action of a vapor pressure produced from a portion of the liquid to be delivered. The pump comprises a closed reservoir for liquid, which includes an unidirectional liquid inlet, an unidirectional liquid outlet, a vapor exhaust valve adapted to balance the pressure between the unidirectional liquid inlet and the reservoir during its filling. The pump also comprises a vapor generator for producing vapor inside the reservoir at a pressure sufficient to force out the liquid contained therein through the liquid outlet, and a control device for operating the vapor generator only when the liquid fed by the liquid inlet has reached a predetermined level in the reservoir.