Activating supports may be suitably prepared by the following procedure (a) providing a porous mineral oxide support material, (b) treating the support with a phosphorus-containing compound, (c) treating the support from step (b) with an organometallic compound, (d) heating the functionalized support from step (c) under an inert gas and then under an atmosphere comprising oxygen, (e) fluorinating the support with a fluorinating agent, and (f) recovering an activating support. The activating supports are suitable used in combination with single site catalysts for the polymerization of olefins. The supports are most preferably used in combination with metallocene complexes. The preparative route for the activating supports provides for supported polymerization catalyst systems having excellent activities.