Sintered YBaSrCu 3O y was treated at 300-950°C under various oxygen pressures of PO 2=0.2-5×10 4 atm. The structure as observed by powder X-ray diffraction (XRD) changed with increasing oxygen content from tetragonal ( y⩽6.6) to orthorhombic ( y⩽7.0) to tetragonal ( y⩾7.0). The maximum orthorhombic distortion and the highest superconducting transition temperature where reached at y=6.90-6.95. The orthorhombic structure for y=6.95 refined by a Reitveld analysis of the XRD data was essentially the same as that of YBa 2Cu 3O 6.9. On the other hand, transmission electron microscopic studies of the latter tetragonal structure strongly suggested ordering of Cu(1)-O chains in microdomains with sizes of 50×50 Å2, the XRD “tetragonal” symmetry being only an averaged one. The transition temperature, Tc, was 82 K at y=6.90, while 80 K at y=7.20. The concentration of carrier holes as estimated from the Hall effect was 4.9×10 21 cm -3 at y=6.90 and 3.9×10 21 cm -3 at y=7.20 at 100 K. Excess oxygen has been supposed to be concentrated in microdomain boundaries where Cu ions get octahedrally coordinated, leading to the reduction of the carrier concentration.