SUPERCONDUCTIVITY, LOCALIZATION AND CRYSTALLOGRAPHIC PHASE TRANSITION IN La_{2 - X}Sr_XCuO_{4 - Y}.

We report a resistive investigation of superconductivity in the system La//2// minus //xSr//xCuO//4 minus //y on samples which have been analyzed by thermogravimetric techniques for oxygen deficiency determination. A maximum superconductive transition temperature is observed in La//2// minus //xSr//xCuO//4// minus //y near the tetragonal-orthorhombic phase transition, together with a resurgence of high temperature superconductivity in non-substituted La//2CuO//4// minus //y. Drastic localization effects are observed in the vicinity of x equals 0. 04 with no sign of superconductivity at low temperature.