High resolution NMR data for the changes in the shift and relaxation time of the 89 Y resonance are presented for a range of zinc and gallium replacements for copper in YBa 2 Cu 3 O 7−δ . Strikingly different variations in these NMR parameters are detected for the two substitutions. The interpretation of the experimental data leads to the conclusion that the incorporation of non-magnetic zinc impurities in the Cu(II)-O plane produces fluctuating, localised magnetic moments which may be the origin of the destruction of the superconducting state.