Isothermal mixing in multicomponent gas media is a highly nonlinear process in which molecular transport can give way to solutal gravitational convection, leading to significant changes in the intensity of mass transfer. This transition is investigated in the present work using the example of the system 0.360 C₃H₈ + 0.640 N₂O — CO₂, the difference in the molar masses of whose components does not exceed 0.2%: the calculated molar mass of the upper mixture exceeds that of carbon dioxide by only 0.078%, which eliminates stratification caused by the difference in molecular masses and allows the role of purely diffusive instability to be isolated. Three-dimensional unsteady-state modeling was performed using the Species Transport approximation (ANSYS Fluent), followed by a comparison with experimental data from a two-flask experiment conducted in the 0.2–0.85 MPa pressure range at T = 298 K. The discrepancies between the calculations and the experimental results are alternating in sign, and the mean algebraic deviation does not exceed 1.3%, indicating the absence of systematic model bias. It is shown that the instability stems from a density inversion caused by the difference in component compressibility: at p ≈ 0.28 MPa, the upper mixture becomes denser than the underlying gas, with the relative density excess reaching 2.8% at 0.85 MPa. The corresponding Rayleigh number changes sign near this same pressure and rises to ~3·10⁶, a result consistent with the observed delay time for the onset of convection (t ≈ 400–600 s). An explanation is proposed for the non-monotonic pressure dependence of transport.