A seal for providing a fluid tight seal between a pair of relatively rotatable components (10, 11) has a first seal face member (25) mounted in fixed axial and rotational relationship and sealed with respect to one component (10) and a second seal face member (45) sealed with respect to said other component (11), said second seal face member (45) being sealed with respect to its associated component (11) by a radially extending diaphragm (30), said diaphragm (30) being disposed on the high pressure side of the seal face members (25, 45), so that when subjected to fluid under pressure the force acting on the diaphragm (30) will urge the second seal face member (45) into engagement with the first seal face member (25).