The invention provides a levelling system for the chair of a stairlift, a characteristic feature of the system being the provision of a safety back-up or locking mechanism that will lock the position of the chair in the event of failure within the levelling drive. This locking mechanism preferably includes a locking wheel that is in partial mesh with a geared wheel forming part of the levelling drive. In the event of failure within the drive the locking wheel meshes to lock the position of the chair.