Abstract
We present SQUID susceptibility data on Zn and Ni substituted YBa2Cu3O6 + x. Cross-checks with NMR yield an unprecedented accuracy in the estimate of the magnetic susceptibility associated with the substituents, from the underdoped to the lightly overdoped case. This allows us to determine the Weiss temperature θ for YBCO: its value is very small for all hole dopings nh. Since in conventional metals the Kondo temperature TK < θ, magnetic screening effects would not be expected for T >> θ; in contrast, increasing nh produces a reduction of the small moment induced by Zn2+ and a nearly constant effective moment for Ni2+ corresponding to a spin 1/2 rather than to a spin 1.