Experimental data are presented for the diffusion of Ar, Kr, Xe, O₂ and N₂ in 4A and 5A zeolites. In the 4A zeolite these gases show a similar pattern of behaviour to that previously observed for simple polyatomic molecules in the 5A sieve but the behaviour of the monatomic and diatomic molecules in the 5A sieve is different. Transition state theory provides a satisfactory interpretation of the diffusivities for the 4A systems on the assumption that transport occurs by an activated diffusion process which involves only single jumps between neighbouring cavities. In the 5A sieve, at low concentrations within the Henry's law region, the diffusivity is inversely dependent on concentration. This suggests a collisional transport mechanism. A simple model based on the assumption that transport occurs mainly through a few rapidly moving molecules, which can traverse several cavities in a single flight, provides a semi-quantitative interpretation of the observed behaviour. This difference in the diffusion mechanism appears to be related to the magnitude of the energy barrier at the sieve window.