Abstract: A support system, for mounting enclosures on hollow poles of the type located on a highway right-of-way and having a range of diameters and wall thicknesses, has a bracket plate which is fastened to the back of the enclosure with mounting subassemblies inserted through holes in the bracket and in the pole wall. Each subassembly comprises a slotted expandable tube, a locking bolt and a special nut. Interaction of the nut and bolt cause expansion of the tubular member, locking the bracket to the wall of the pole. Also in the bracket and aligned with each subassembly, are two angled, threaded apertures which retain stabilizing screws. Subsequent to the locking action, the screws are advanced into forceful contact with the exterior of the pole wall. An enclosure back portion is then mounted on the bracket by hexagonal head bolts or the like, the heads of the bolts being seated in hexagonal depressions in the rear of the brackets for inaccessability from the outside of the cabinet.