Aromatic nitration has been used to modify the polymer main chain of polysulfone and polyethersulfone. Alteration in the gas separation characteristics of homogeneous membrane made out of these polymers could be observed upon nitration. Generally, separation factors for gas mixtures of CO2/CH4 increased while the permeability decreased. Remarkably, a decreasing diffusivity for both gases upon nitration could not be correlated with an increasing free-volume but was interpreted as an drastically decreased main-chain flexibility. The decrease of main-chain segmental mobility could be verified experimentally by a diminished amount of ß-relaxations determined by torsional braid analysis.