The gas permeation properties of a series of new poly(ether ketone)s (PEKs) have been measured and correlated with the chemical structure. Polymers with hexafluoroisopropylidene (6F) groups in the backbone present a better permeability than those with sulfone (SO2) groups, due to a significant increase in diffusivity caused by an increase of free volume. The introduction of bulky groups also causes an increase in permeability, that, in this case is produced by a simultaneous improvement of the diffusivity and the solubility. The behaviour of these polymers can be correlated with the mobility of the chains as determined by DMTA and molecular modelling.