Mixtures of monocyclopentadienyltitaniumtrichloride derivatives [Cp'TiCl3, Cp' = C5H5 = CP (1), C5Me5 = Cp* (2), Me = CH3] and methyl-aluminoxane (MAO), well known catalytic systems for olefins, styrene and dienes polymerisation, were characterised by ESR spectroscopy. The assignment of the observed signals to defined structures was carried out by comparison with the systems: 1, 2/AlMe3; 1/Al(iso-butyl)(2)hydride, 2/AlEt3, 2/AlMe3/[CPh3][B(C6F5)(4)]. In all the analysed mixtures titanium is partially reduced to the trivalent state and, generally, 1 shows a higher tendency to reduction than 2. At low Al/Ti ratio (R = 10) both in the presence of AlMe3 or MAO, 1 and 2 produce similar Ti(III) compounds: the bimetallic trinuclear complex Cp'Ti[(mu-Cl)(2)AlMe2](2) (A and A* for 1 and 2, respectively). At higher ratio (R = 300, 500), new species are observed, at higher g-values, compatible with a alkylation of the metal centre. In the system 1/MAO a rather unstable new species C appears initially at g = 1.977, and is progressively converted to a stable titanium hydride compound (D). In the system 2/MAO (R = 300, 500) the species C* (g = 1.975) and C-1* (g = 1.977) are observed. C* and C-1* form in the system Cp*TiCl3/AlMe3/[CPh3][B(C6F5)(4)] as well, i.e. in the presence of the ionising agent [CPh3][B(C6F5)(4)], which is also employed to activate the half-metallocene precursors for styrene polymerisation, pointing to the formation of common species in the two catalytic systems. (C) 2004 Elsevier B.V. All rights reserved.