[en] Using samples of lunar material the isotopic composition of inert gases during solar flares is analyzed. Three samples of lunar meterials are studied: 24092.4-1.16, 24118.4-1.13 and 24143.4-1.12. A complex multi-component composition of inert gases of the solar type corresponding to different solar activity is observed. Gases of the SF type characterize the isotopic composition of gases emitted by the Sun during solar flares. This type of gases is depleted of light isotopes. The SW type characterizes the solar wind isotopic composition in the quiet solar period, the SE type is enriched with light isotopes with respect to the SW type. It is found that the isotopic compositions of SF, SW, SE type gases follow the multi-step mass fractionation law. A model of isotope fractionation processes in the Sun is proposed