The effect of annealing in high-pressure oxygen on YBa2(Cu1-xMx)3Oy(M=Co, Ni, Zn) was studied. The Tcstructure and oxygen content of Ni- and Zn-doped samples were little changed by annealing in high-pressure oxygen, while Co-doped samples were found to have transition temperatures that increased with increasing oxygen content. This difference is attributed to the extra oxygen atoms introduced into the microdomain boundaries by annealing in high-pressure oxygen. The extra oxygen atoms can “smooth” the microdomain boundaries and increase the [Cu-O]+hole concentration, thus improving the superconductivity. © 1988 The Japan Society of Applied Physics.