New approaches have been identified for the immobilization, on a MgCl2-based support, of borate activators for metallocene-catalyzed olefin polymerization. Immobilization of [HNEt3][B(C6F5)3(C6H4-4-OH)] was carried out by reaction with a support of type MgCl2/AlRn(OEt)3-n, obtained by reaction of AlEt3 with an adduct of magnesium chloride and ethanol. Use of the resulting immobilized borate in combination with zirconocene catalysts in ethylene and propylene polymerization resulted in significantly higher polymerization activities than were obtained using the same borate immobilized on a silica support. The MgCl2-based support also gave the better polymer particle morphology, cross-sectional imaging indicating uniform support fragmentation, as opposed to incomplete fragmentation using the silica support. High catalyst activities were obtained using a MgCl2/AlRn(OEt)3-n support impregnated with [Ph3C][B(C6F5)4]. In this case, a highly porous polyethylene particle morphology was obtained.