Systematic investigations of phase systems and transitions are the foundation of understanding complex physical phenomena such as superconductivity. One example in which the lack of such a systematic knowledge has led to contradictory results is β-Fe1+xSe. The superconductivity of this “simple” two-dimensional, layered structure has been reported to be heavily dependent on the composition and closely related structural transitions, defects and various superstructures with overall inconsistent results. The difficulties arise from the narrow phase width of β-Fe1+xSe (0.01 < x < 0.04), a small deviation from the 1:1 ratio and its manifold of neighbouring phases. We report the precise post-synthetic control of the composition of β-Fe1+xSe by electrochemistry with simultaneous tracking of the concomitant structural changes via in-situ synchrotron X-ray diffraction. Via an externally applied potential, electrochemistry allows us to confirm the phase width of 0.01 < x < 0.04. We identify the superconducting state below 8 K, which in contrast to earlier reports is independent of the composition. However, in a second set of in situ X-ray diffraction experiments, we demonstrate that β-Fe1+xSe forms a new phase in the presence of oxygen above a 100 °C, which has the same anti-PbO type structure but is not superconducting down to 1.8 K. This electrochemical approach takes experiments for two-dimensional materials beyond intercalation reactions, and exploits the exquisite control provided by electrochemistry.