We systematically investigated the peritectic decomposition temperatures (T_p) of (Sm_1-xGd_x) Ba₂Cu₃O_y solid solutions and superconducting properties (T_c and J_c-B) of binary mixed (Sm_1-xGd_x)-Ba-Cu-O bulk superconductors melt-processed in a 1%O₂-Ar atmosphere. The T_p of (Sm_1-xGd_x) 123 linearly decreases from 1, 023 to 1, 003°C with increasing x (i.e., the amount of Gd substitution on Sm). The cooling rate of x=0 and 1 needed to fabricate a single-domain bulk is higher than that of x=0.25, 0.50 and 0.75. Although the T_c (93.5-94K) of melt-textured (Sm_1-xGd_x) BCO is independent of x and the Pt dopant, J_c values increase with Pt doping since the (Sm_1-xGd_x)₂Ba₁Cu₁O_y is refined.