Abstract The phase relation and crystallization behavior were investigated in a series of (Sm 0.5Eu 0.5) 1+ x Ba 2− x Cu 3O 7− δ (−0.1⩽ x⩽0.3). The single phase rare earth (RE)–Ba substitution window (RE: Sm and Eu) was found to range from 0 to 0.3 when quenched from 1010 °C for nominal substitution parameter, x⩽0.05, from 990 °C for x=0.1 and 0.2, and from 970 °C for x=0.3 under ambient atmosphere. Onset T c values were 94 K for x⩽0 and fell sharply with increasing x. After Ar post-annealing (ArPA), the countersubstitution of RE by Ba was observed associated with a slight improvement of onset T c (0.5 K). A dramatic enhancement of onset T c for the sample with RE rich composition ( x>0) was observed with ArPA. It was also found that crystallization rate was large for the compositions with x⩽0 while small for x>0.