The pore size, shape and orientation of an illite-dominant clay were mapped during one-dimensional compression, using mercury intrusion porosimetry (MIP), scanning electron microscopy and gas adsorption. The total porosity was found to spread over the three International Union of Pure and Applied Chemistry classes of pores sizes: micropores (below 2 nm), mesopores (2–50 nm) and macropores (above 50 nm), and all three pore classes were observed during the compression. The clay structure is aggregated, with visible inter-aggregate pores (about 80% of the total porosity), and the remaining intra-aggregate pores of size approximately equal to the thickness of illite platelets (50–100 layers). During compression the largest pores first collapsed, followed by a progressive collapse, in an orderly manner, of smaller and smaller pores. MIP data suggest that the macroscopic deformation mainly translates at the pore scale into changes of inter-aggregate porosity, while intra-aggregate pores spread over the micro- to mesopore size range. Gas adsorption tests show that the volume of intra-aggregate pores decreases with loading, probably due to rearrangement of particles composing the aggregates, while the specific surface area reduces. Examination of the pores’ orientation on both vertical and horizontal planes confirms a preferential orientation of pores normal to the loading direction, with a gradual flattening of the pores.