Supported titanium-magnesium catalysts (TMC) comprising isolated and clustered titanium ions in different oxidation states, which are obtained using titanium compounds of different composition (TiCl4, TiCl3 center dot nDBE (DBE - dibutyl ether), [n(6)-BenzeneTiAl(2)Cl(8)]), were synthesized and tested in ethylene polymerization. The state of titanium ions was studied by the ESR method both for the procatalysts and after their interaction with triisobutilaluminum. For identification of ESR-silent Ti3+ ions and Ti2+ ions, special procedures of additional catalyst treatment with pyridine, water, and chloropentafluorobenzene were used to obtain Ti3+ ions that are observable in ESR spectra. In distinction to numerous earlier works performed with the TiCl4/MgCl2 catalyst comprising after the interaction with AlR3 the Ti3+ surface compounds both as isolated ions and clusters (ESR-silent), this work considers the [eta(6)-BenzeneTiAl(2)Cl(8)]/MgCl2 catalyst (TMC-3) comprising mainly the isolated Ti2+ ions and a new catalyst TMC-4 obtained by treating the TMC-3 with chloropentafluorobenzene. This catalyst comprises only the isolated Ti3+ ions both before and after the interaction with triisobutyl aluminum. It was shown that in spite of sharp distinctions between the catalysts under consideration concerning titanium oxidation state and the ratio of isolated Ti3+ ions to clustered ones, all these catalysts produce polyethylenes with similar molecular weights and molecular-weight distributions. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6362-6372, 2009