The swelling behaviour of expansive clays is strongly related to the interaction between clay particles. In this paper, a series of constant-volume swelling pressure and mercury intrusion porosimetry (MIP) tests were carried out on MX80 bentonite/Callovo-Oxfordian (COx) claystone mixtures with different bentonite fractions and dry densities. Results show that the swelling pressure increased linearly with the increase of dry density. Additionally, at a given dry density, the larger the bentonite fraction, the larger the swelling pressure. From the MIP results, the inter-particle pore volume decreased with the increases of bentonite fraction and dry density. To link the macroscopic swelling behaviour with the microstructure features, a new method was proposed, allowing the determination of the average inter-particle distance from the pore size distribution. Moreover, a linear relationship was identified between the swelling pressure and the average inter-particle distance in a semi-logarithmic plane, regardless of the bentonite fraction and montmorillonite content.