The behavior in propylene polymerization of divalent titanium compounds of type [h 6 -areneTiAl 2 Cl 8 ], both as such and supported on activated MgCl 2 , has been studied and compared to that of the simple catalyst MgCl 2 /TiCl 4 . Triethylaluminium was used as cocatalyst. The Ti-arene complexes were active both in the presence and in the absence of hydrogen, in contrast to earlier reports that divalent titanium species are active for ethylene but not for propylene polymerization. 13 C-NMR analysis of low molecular weight polymer fractions indicated that the hydrogen activation effect observed for the MgCl 2 -supported catalysts should be ascribed to reactivation of 2,1inserted (''dormant'') sites via chain transfer, rather than to (re)generation of active trivalent Ti via oxidative addition of hydrogen to divalent species. Decay in activity during polymerization was observed with both catalysts, indicating that for MgCl 2 / TiCl 4 catalysts decay is not necessarily due to overreduction of Ti to the divalent state during polymerization. In ethylene polymerization both catalysts exhibited an acceleration rather than a decay profile. It is suggested that the observed decay in activity during propylene polymerization may be due to the formation of clustered species that are too hindered for propylene but that allow ethylene polymerization.