Abstract: A pressure modifying valve for anaesthetic gas scavenging systems for connection between a high vacuum source such as a hospital building service and the outlet from a patient or anaesthetic machine. The valve comprises a housing divided by a partition into two chambers having inlet and outlet ports respectively. The partition has a flow restricting orifice controlled by a valve member which is supported on a diaphragm to be self adjusting in response to pressure in the chamber. On the patient side, the pressure in chamber is controlled by pressure relief valves which are similar and interchangeable. The valve is made inexpensively of plastics and is disposable before the plastics deteriorates or undesirable matter accumulates and affects the functioning of the valve.