Abstract: A sample port valve for use in a port of a liquid-containing vessel. The valve incorporates a housing that is attachable to a port of the vessel, such as the permeate port of a reverse osmosis pressure vessel. A preferred valve allows a user to insert and retain a sampling tube into the valve to a position less than fully inserted. With the tube in its inserted, retained position the valve can then be fully opened to allow the tube to be fully inserted into the vessel, without the loss of liquid or pressure from the vessel.