Abstract
The detailed characterization of YBa2(Cu1-xCox)3Oy has been made for various x values. The critical temperature is found to be constant in the small x region and then decrease linearly with x and abruptly vanish at a critical concentration. This result indicates a very different role of magnetic impurities in this compound compared with conventional superconductors. Two transition stages, which were independent of Co concentration, were observed in the ac susceptibility. This is concluded to be an intrinsic character of the Co substitution system. The results are discussed in comparison with other substitution systems.