Importance of mass transfer coeffs. in air separation both on sorbents (carbon and zeolite molecular sieves) and by pressure-swing adsorption was evaluated. In particular, the role of mass transfer coeffs. in vacuum-swing adsorption on zeolite molecular sieves VSA-II (UOP prods. in prodn. of O was taken into consideration. The increase in the mass transfer coeffs. for O results in an increase in the process efficiency.