Hydraulically operated camshaft phasers are described which comprise a drive member and a driven member having a fixed range of angular adjustment and a means for mechanically locking the position of the driven member relative to the drive member when there is insufficient hydraulic pressure to operate the phaser. In the invention, the lock operates in an intermediate position within the range of adjustment, and a hydraulic circuit is provided for biasing the phaser towards the intermediate position for the lock to engage.