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Dopant site occupancies in YBa2Cu3 -"M"O7 -s with Af Fe (x 0.3 and x 0.5), Co (x 0.2 and x=0.5), Ni (x 0.3), and Zn (x 0.3) have been found using differential anomalous x-ray scattering. The Ni and Zn atoms were found to occupy the Cu(1) ("chains") site and the Cu (2) (" planes" ) site in a nearly random distribution. The Fe and Co atoms were found to occupy the Cu(1) site predominantly at low x, with an increasing fraction on the Cu(2) sites as the total amount of dopant increases. In all cases, our results appear to have high statistical significance, with very little sensitivity to expected uncertainties in oxygen content, total dopant content, anomalous corrections to the atomic scattering factor of the dopant, and to relative atomic coordinates assumed in the modeling. %e have also discussed the results in the context of existing extendedx-ray-absorption fine-structure and neutron-diffraction results, thermogravimetric analysis, Mossbauer spectra, and T, and Hall-effect studies.

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