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The superconducting and magnetic properties of Fe y Se 0.25 Te 0.75 single crystals ͑0.9Յ y Յ 1.1͒ were studied by means of x-ray diffraction, superconducting quantum interference device magnetometry, muon-spin rotation, and elastic neutron diffraction. The samples with y Ͻ 1 exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for y Ն 1 but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the Fe y Se 1−x Te x can be tuned not only by varying the Se/Te ratio but also by changing the Fe content.