The catalyst precursors Tp(Ms*)V(L)Cl-2 [1, L = N'Bu; 2, L = 0; Tp(Ms*) = (3-mesityl-pirazolyl)(2)(5-mesityl-pirazolyl)] were in situ supported onto SiO2 and onto methylaluminoxane (SMAO-4) and trimethylaluminum (STMA-3) modified silicas using 0.02 wt.% V/support. All catalyst systems were shown to be active in ethylene polymerization, with activities lying between 1000 and 1900 kg of PE/mol [V] h atm. Similar activities were found by employing different types of cocatalysts (MAO, TMA, and IPRA) in the in situ polymerization reaction using 1/SMAO-4. Furthermore, the systems were shown to be stable at different [Al]/[V] molar ratios and polymerization temperatures. Polyethylenes were characterized by differential scanning calorimetry (DSC). The impossibility of dissolving them in a common solvent for GPC analysis suggests that such polymers might present very high molecular weights. (c) 2006 Elsevier B.V. All rights reserved.