Display omitted •Binary and ternary gas diffusion is investigated in situ with FTIR Spectroscopy.•The binary diffusion of CH4/CO2, CH4/C2H6, CH4/C3H8 and C2H6/C3H8 is studied.•Ternary gas diffusion of CH4/CO2/C3H8 is measured at 0.1 MPa and 308.15 K.•The Maxwell – Stefan theory of multicomponent diffusion is effectively applied.•Osmotic, reverse and barrier diffusion are observed and predicted. The unsteady diffusion of the ternary gas mixture CH4(α) – CO2(β) – C3H8(γ) is measured with FTIR Spectroscopy in the transmission mode at 308.15 K, 0.1 MPa and equilibrium compositions xα,∞=0.333 and xβ,∞=0.363. A parallel is drawn between the experimental approach and the classical Two-Bulb method. The geometry of the setup is calibrated by measuring the equimolar counter-diffusion of C3H8/CO2 at 308.15 K and 0.1 MPa. Then, analogous experiments are conducted for the gas pairs CH4/CO2, CH4/C2H6, CH4/C3H8 and C2H6/C3H8 to measure their mutual diffusivity. Last, the ternary diffusion of CH4(α) – CO2(β) – C3H8(γ) is measured and modelled with the Maxwell – Stefan theory of multicomponent equimolar diffusion for ideal gas mixtures. Osmotic, reverse and barrier diffusion are observed and predicted for methane before reaching equilibrium.