Abstract: Mssbauer spectra measurements reveal that Sn exists in YBa_2Cu_(3_x)Sn_xO_(7_y) system (x=0.05,0.10,0.20,0.30 and 0.40)in form of Sn~(4+) and occupies Cu(1)and Cu(2)sites.The relative rate at which Sn~(4+) occupies Cu(1)or Cu(2)sites varies with x.The superconductivity transition temperature,T_c,depresses slightly due to the substitution of Cu for Sn.

Key words: Y-Ba-Cu-O system, M(?)ssbauer spectrum, substitution, superconductivity transition temperature