The atomic structure of the high-temperature β-Fe6Ga5 phase, formed in the Fe-45at.%Ga alloy during rapid cooling from the melt, has been refined by X-ray, neutron diffraction techniques, and density functional theory calculations. The regularities of the phase composition evolution of the quenched Fe-45at.%Ga alloy in the process of continuous slow heating and cooling under vacuum conditions have been revealed. It is shown that after the heating-cooling cycle, the structures of the outer layers and the sample volume are completely different.