A systematic study is reported on the range of stability of molybdenum substituted Sr-based 123 compounds with Mo-Sr-Y-Cu system, synthesized under ambient pressure. All materials crystallize in the space group: P4/mmm and the observed solubility limit of Mo is rather low in this structure because secondary phases start to form as soon as x>0.3 in nominal composition is reached. The antiferromagnetic property in all superconducting multiphase samples can be attributed to the secondary phase Y 2Cu 2O 5. The influence of oxygen annealing on the changes in electronic structure for the pure Mo 0.3Cu 0.7Sr 2YCu 2O y system associated with a non-superconductor to superconductor transition has been investigated by means of X-ray photoelectron spectroscopy, powder X-ray diffraction, magnetic susceptibility and specific-heat measurements. We unambiguously show the predominance of the Mo 5+ state over the Mo 6+ one on both as-synthesized and annealed phases; annealing under an oxygen atmosphere enhances both the Mo 6+ and Cu 2+ amounts. We suggest that the enhancement of Mo 6+ under oxygen annealing is in close relation with the decrease in the O 2p→Cu 3d charge-transfer energy resulting in superconducting properties.Read less <