Diffusion of carbon dioxide in 5A zeolites (laboratory-synthesised and commercial powders, pressed and manufactured pellets) has been investigated using the frequency-response method. Intra- and intercrystalline diffusion processes can be distinguished and determined from the response spectra generated. Effects of mass and heat transfer resistances on the response spectra have been observed, but could be eliminated by dispersing a small amount of sample in a plug of glass-wool and carrying out measurements at relatively high temperatures.