Oxy-anion (PO3- 4) substituted phases of the composition REBaSrCu2.8(PO4)0.2Oy have been synthesized. Single phase formation is achieved only for the larger rare earths (La → Ho and Y) and all the phases crystallize in the tetragonal 123 structure. IR studies confirm the presence of a phosphate group in the structure. The total oxygen content (including that associated with the oxy-anion PO3- 4) has been found to be in the range 7.01 to 7.10 per formula unit. Except for RE = La, Nd, Yb and Lu, all the phases exhibit superconductivity and the Tc,zero values vary from 18 to 38 K. The 7c,zero increases with decreasing size of RE3+. ErBaSrCu3O7 is multiphasic and single phase formation is achieved for the first time by phosphate substitution at the copper site. Single phase formation is encountered for x = 0.10 and 0.15 with Tc,zero values of 54 and 37 K respectively. © 1997 Elsevier Science B.V.