We report magnetic susceptibility data from 4 to 290 K for four supported Ziegler-Natta model catalysts of general formulation SiO 2 + xAMB + γTiCl 4, where AMB is an alkylmagnesium butoxide reagent. For each variation of the formulation, the spin concentration measured by magnetic susceptibility is nearly equal to the spin concentration measured by electron spin resonance (ESR) spectroscopy. By both methods we obtain [spins/Ti] values between 4.0 and 20.5%. For the three most reduced catalyst preparations studied—1.2, 1.3, and 1.4 ( 1 < x y < 10 )—these values are three to nine times lower than the [ Ti(3+) Ti(total) ] ratios found for the catalysts by electrochemical methods [measurements of Ti(2+) concentration by hydrogen evolution show that Ti(2+) is negligible in these formulations]. The near equivalence of [spins/Ti] values measured by ESR and magnetic susceptibility and the deficiency of these values compared with Ti(3+) measured electrochemically establish that the reduced formulations contain clusters of Ti(3+) ions, [Ti(3+)] n where n > 1. Apparently, in those cases where n is even, the titanium (3+) ions in the clusters are spin paired, whereas where n is odd, there is one unpaired electron per cluster. It has been shown previously by ESR that TiCl 4 can bind to these reduced clusters and render them ESR active. Now, the magnetic susceptibility data also suggest that, on binding to the [Ti(3+)] n clusters, TiCl 4 breaks Ti(3+)-Ti(3+) bonds. Thus, in the case of the most oxidized formulation 1.1 ( x y = 0.55 ), which contains only 9% Ti(3+) by electrochemical determination, a much higher fraction of the Ti(3+) (65–95%) is detected by ESR and magnetic susceptibility.