This work studied the critical pore dimensions for different light gases in disordered microporous hybrid silica which was derived from 1,2-bis(triethoxysilyl)ethane (BTESE) precursor. The Lennard-Jones (LJ) parameters of the material, obtained from high pressure isotherms of light gases, were employed to evaluate dispersive interaction, diffusivity and adsorption coefficient in the nanopores of the hybrid silica, and the results demonstrated that the entrance barrier was the key factor to break down the separation limits confining the Graham law and the Knudsen model by gas kinetic theory. Finally, the critical pore dimensions for different resistance dominated regions were evaluated, which theoretically explained why the separation capacity of the hybrid silica was less promising than pure silica for the purpose of gas separation.