This paper describes an experimental study aimed at evaluating the influence of soil microstructure on air permeability in compacted clay. Air permeability measurements, estimated using the gas pressure decay method, were carried out for a wide range of compaction states. The evolution of the air permeability during wetting and drying paths was also evaluated. The experimental results show that, for an increase in the as-compacted degree of saturation, air permeability may either increase or decrease depending on the as-compacted dry density. Air permeability increases with increasing the degree of saturation in loose specimens, whereas the opposite trend is observed for dense specimens. Microstructural analysis, carried out using Mercury Intrusion Porosimetry (MIP) tests, shows a strong dependency of the air permeability on the as-compacted soil microstructure. This behaviour is also noticed in specimens that experienced a large variation in the degree of saturation during wetting and drying. Microstructural data indicate that air permeability is mainly controlled by large pores that display high connectivity. The degree of saturation plays a dual role in soil microstructure which, in turn, affects the air permeability. Denser specimens (dry density = 1.5 Mg/m3) show a reduction in keff due to the expansion of the clay aggregates with increasing the as-compacted degree of saturation. The increase in the as-compacted degree of saturation in loose samples (dry density = 1.3 Mg/m3) produces an enhancement in the proportion of macro pores, thus increasing keff, as a consequence of modifications in the PSD. A threshold value has been identified, above which further increase in degree of saturation causes a reduction in the proportion of macro pores, and therefore in keff. A new proposal for estimating the air permeability is proposed in this paper based on the determination of a pore size parameter (PSP) obtained from MIP data. The proposed approach seems capable of describing the evolution of air permeability for the whole spectrum of compaction states, including specimens subjected to wetting and drying paths.