Abstract: The invention concerns an air transfer valve which automatically vents accumulated air from pressurized liquid reticulation pipelines or vessels. The valve (10) has a housing (12) which is connectable to the pipeline or vessel. The housing has a first outlet (38) venting to atmosphere and a control chamber (60) which is exposed to internal pressure in the housing via a control chamber inlet. A first valve closure (34) can move to open and close the first outlet. This valve closure is exposed to control chamber pressure tending to move it to close the first outlet and to internal housing pressure tending to move it to open the first outlet. When the housing is pressurized the first valve closure (34) is maintained in a closed position by virtue of an unbalanced pressure force acting on it that is attributable to exposure of the valve closure to atmosphere through the first outlet. There is also a control chamber outlet (36) from the control chamber (60) to atmosphere.