The measurements of x‐ray diffraction, the temperature dependence of the dc resistance and the ac susceptibility have been performed for the single‐phase 3D‐metal doping systems YBa2 Cu3−x Mx Oy (M=Fe, Co, and Ni; x=0.025, 0.05, 0.075, 0.10, 0.25, and 0.50 for Ni and Co and 0.05, 0.075, 0.10, 0.15, and 0.20 for Fe). With an increase of impurity content, two structural transitions were observed for the Co and Fe dopants but only one for the Ni dopant. The resistivity in the normal state changes from metallic to semiconductinglike behavior and the depression of Tc is linear with the impurity concentration (x) when x<0.10. A weak Curie–Weiss type paramagnetism, which is enhanced with impurity content, exists in the samples studied. Incorporating other work on oxygen defects, we suggest that a change of oxygen content induced by doping was the dominant effect on superconductivity in these samples.