The features of the change in the regimes "diffusion-gravitational concentration convection" in isothermal ternary gas
mixtures have been studied experimentally and numerically. It is shown that, at a certain pressure, due to the difference
in the diffusion coefficients of the components, an instability of mechanical equilibrium arises, leading to the appearance
of structured convective formations. Calculation results are compared with experimental data on the location of the
instability boundaries in the studied gas mixtures. Intensity of convective mixing is also influenced by the content of the
component with the highest molecular weight in the initial mixture. In terms of Rayleigh numbers, a boundary relation is
obtained that determines the regime change. The solution of the problem of multicomponent mixing in a cylindrical
vertical channel of finite height with solid and impermeable boundaries for the substance made it possible to determine
the concentration distribution along the length.