Changes in the superconducting behaviour of Y-124 is examined using cobalt and nickel atoms as dopants at the copper sites. Cobalt substitution destabilizes the Y-124 structure and deteriorates the superconducting properties dramatically. Conversion of the CuO double chain into a single one is caused by a quite small amount (3 at.%) of Co substitution. The rate of suppression of T c for Co and Ni doped Y-124 samples is appreciably larger than in Y-123 samples with similar dopings. An orthorhombic to tetragonal transition is observed for Co doped samples at optimum concentration (3 at.%). In contrast, the structure of Ni doped Y-124 remains orthorhombic at all levels of dopant concentration. The difference in behaviour of Co and Ni doped samples is due to their different site preference namely [Cu(I)] for the former and [Cu(II)] for the latter.