A mounting structure for mounting a support for a seat belt to a vehicle, arranged so that when mounted to a vehicle it will move relative to the vehicle in a first range of directions under application of a first force urging the mounting structure in one of the range of directions, but will not move relative to the vehicle in a direction outside the first range of directions when a second, greater, force is applied urging the mounting structure in such a direction. The structure may have a bracket (7) secured to a vehicle by bolts (18) passing through slots (15) in the bracket, allowing the bracket to move generally in the direction of the slots, but not otherwise. De-couplers, in which toothed flanges (20) on collars (19) through which the bolts extend bear against raised pads on u-shaped washer structures (23) may be disposed between the bolt heads and the bracket to limit the force applied to the bracket and enable the applied force at which the bracket will move relative to the vehicle to be controlled.