There is described a powder isolating valve which is capable of sealing valves from powder leakage during transport of powder and/or granular material. The valve comprises a rotatable valve closure member (14) and a pneumatically activated sealing mechanism containing a pair of resiliently deformable structures (18,20) extending around a surface of the rotatable valve closure member when the latter is in its closed position, the pair of resiliently deformable structures being held in a pair of annular recesses (32,34) which also extend around an surface of the rotatable valve closure member in its closed position, wherein the pair of resiliently deformable structures are capable of being pneumatically activated to force a sealing ring (26) against the rotatable valve closure member.