Abstract. The electric properties of the system YBa_2-xSr_xCu₃O_7-y have been studied in different regimes of preparation. After multiple annealing of the powders and firing of the pellets in oxygen at x = 1 a superconductor with T_c = 88 K and ΔT_c = 1 K was prepared. After a single annealing only of the pellets, the following results were obtained: for 0 ≤ x < 1 the samples are superconductors, the amount of superconducting phase being above the percolation level. For 1 ≤ x ≤ 1.5 the amount of superconducting phase is below the percolation level. For 1.7 ≤ x ≤ 2 the samples obtained are not superconducting down to 4.2 K. Some possible reasons for the observed decrease in T_c with x are discussed.

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