Single-gas permeabilities, selectivities and activation energies of permeation of thin (similar to 0.5 mu m), intermediate and thick (similar to 25 mu m) amorphous glassy polymer films of a polysulfone (Udel 1700) and a polyimide (6FDA-IPDA) were investigated. The films had identical thermal histories and were solvent-free. Evidence of strongly thickness-dependent physical ageing was discovered. Thin films showed decreasing permeabilities and increasing selectivities that surpassed those of intermediate and thick films. Thin films showed increased activation energies of permeation. These results provide evidence for an accelerated approach of thin films to the densified equilibrium state.