Tris(pyrazolyl)borate imido vanadium (V) was immobilized onto a series of inorganic supports: SiO2, MAO-modified SiO2 (4.5 and 23 wt.% Al/SiO2),SiO2-Al2O3,MgCl2,MCM-41 and MgO. Immobilized metal content determined by XRF remained between 0.045 and 0.098 mmol V/g support. The highest metal content were observed in the case of supports bearing large surface area (SiO2-Al2O3 and MCM-41). All the systems were shown to be active in ethylene polymerization in the presence of MAO or TiBA/MAO mixture (AI/V = 1000). Catalyst activity was shown to depend on the support nature, being between 8.1 and 88.0 kg PE/(mol V h). Best results were observed in the case of silica as support. Acid or basic supports afforded less active systems. In situ immobilization led to higher catalyst activity, even in the presence of common alkylaluminum co-catalyst (TiBA). Resulting polyethylenes showed M, higher than 2.7 x 10(6) Da, suggesting the production of ultra-high molecular weight polymers. (C) 2003 Elsevier B.V. All rights reserved.