Sorption and diffusion of carbon dioxide were measured in amorphous and crystalline poly(aryl-ether-ether-ketone) or PEEK, at different temperatures. The sorption follows Henry’s law at all the investigated temperatures, and the reduction of sorption in the crystalline sample corresponds to the reduction of the amorphous phase as derived from X-rays. Due to a more tortuous path, the diffusion parameter in the crystalline sample is lower than in the amorphous one, whereas the activation energy of the diffusion process is similar for both the samples. The permeability to carbon dioxide was very low, comparable, or lower than other rigid polymers.