Treatment of hydroxylated silica or alumina with tris (perfluorophenyl) borane (1), bis (perfluorophenyl) borane (2) or bis (perfluorophenyl) boron chloride (3), provides borane- functionalized supports of variable composition, as revealed by in situ monitoring by 19F NMR spectroscopy and/or elemental analysis. These chemically treated supports can be impregnated with Cp2ZrMe2 to provide supported catalysts for ethylene polymerization. ...