A process for the preparation of a chromium-type supported olefin polymerization catalyst. A fluidized bed of support particles in an inert carrier gas is established. A chromium (III) compound is added to the fluidized support particles to provide a supported catalyst component. The supported catalyst component is activated to convert at least a portion of the chromium (III) to Chromium (VI). The chromium (III) containing particles may be recovered from the fluidized bed and then activated or they may be activated in the fluidized bed. Also the support particles can be treated in the fluidized bed with other treatment agents. The support particles may be pretreated with a solution of a boron treating agent prior to incorporation of the support in the fluidized bed.