A pressure swing adsorption system which includes first and second chambers (110, 112), each containing an adsorption medium, in which when the system is in use a fluid is directed to flow over the adsorption medium in one of the chambers (the "online chamber") in order to adsorb contaminants in the fluid while the adsorption medium in the other of the chambers is regenerated by desorption of adsorbed contaminants (the "regenerating chamber"), the system including: a. a purge conduit which extends between the first and second chambers for delivering fluid when the system is in use from the online chamber to the regenerating chamber, the purge conduit containing a flow restrictor for controlling the volume of fluid which can flow through the purge conduit when the system is in use, wherein the purge stream is vented to atmosphere through an exhaust, controlled by an exhaust valve, the exhaust valve being controlled by means of a piston and cylinder arrangement, where the position of the piston in the cylinder is controlled by means of a flow of pilot gas; b. a re-pressurisation pathway which extends between the first and second chambers for delivering fluid when the system is in use from the online chamber to the regenerating chamber, bypassing the flow restrictor in the purge conduit, c. a re-pressurisation control valve for controlling the flow of fluid in the re-pressurisation pathway, which comprises a housing having a cavity within it with a port (854a, 856a) for flow of fluid along the re-pressurisation pathway and a valve member which can move in the cavity between (i) a closed position in which the valve member engages the port and restricts flow of fluid through the re-pressurisation pathway and (ii) an open position in which the valve member is displaced from the port allowing flow of fluid through the re-pressurisation pathway, and d. an actuator which can act on the valve member to move it from its closed position; e. characterised in that the re-pressurisation control valve is controlled with the piston and cylinder arrangement of the exhaust valve by means of the flow of pilot gas so that when the re-pressurisation control valve is in its open position, allowing gas to by-pass the flow restrictor, the exhaust valve is closed and when the re-pressurisation control valve is in its closed position, preventing gas from by-passing the flow restrictor, the exhaust valve is open. a. a purge conduit which extends between the first and second chambers for delivering fluid when the system is in use from the online chamber to the regenerating chamber, the purge conduit containing a flow restrictor for controlling the volume of fluid which can flow through the purge conduit when the system is in use, wherein the purge stream is vented to atmosphere through an exhaust, controlled by an exhaust valve, the exhaust valve being controlled by means of a piston and cylinder arrangement, where the position of the piston in the cylinder is controlled by means of a flow of pilot gas; b. a re-pressurisation pathway which extends between the first and second chambers for delivering fluid when the system is in use from the online chamber to the regenerating chamber, bypassing the flow restrictor in the purge conduit, c. a re-pressurisation control valve for controlling the flow of fluid in the re-pressurisation pathway, which comprises a housing having a cavity within it with a port (854a, 856a) for flow of fluid along the re-pressurisation pathway and a valve member which can move in the cavity between (i) a closed position in which the valve member engages the port and restricts flow of fluid through the re-pressurisation pathway and (ii) an open position in which the valve member is displaced from the port allowing flow of fluid through the re-pressurisation pathway, and d. an actuator which can act on the valve member to move it from its closed position; e. characterised in that the re-pressurisation control valve is controlled with the piston and cylinder arrangement of the exhaust valve by means of the flow of pilot gas so that when the re-pressurisation control valve is in its open position, allowing gas to by-pass the flow restrictor, the exhaust valve is closed and when the re-pressurisation control valve is in its closed position, preventing gas from by-passing the flow restrictor, the exhaust valve is open.