The paper presents the results of laboratory compaction tests on two samples of the lower Kimmeridge Clay Formation obtained from two different outcrop localities in the UK. The purpose of the testing programme is to study the compaction behaviour of an argillaceous sediment from the same parent material but with different diagenetic histories. Since most available data are for clays subjected to moderate effective stresses (less than 20 MPa), the compaction laboratory tests were carried out to as high as 120 MPa, corresponding to about 9 km burial. The microstructure of the tested materials was studied in a scanning electron microscope before and after compaction testing to evaluate the effects of diagenesis on the microstructure of argillaceous sediments. The two tested materials have, in the past, been subjected to different burial depths and degrees of chemical precipitationand cementation. Several pieces of evidence are shown of the effects of mechanical and chemical processes on the compaction behaviour of argillaceous materials. Mechanical compaction can only account for part of the porosity loss and increase in stiffness of argillaceous materials during burial. Mechanical compaction produces an anisotropic rearrangement of clay particles in uncemented sediments. Once chemical diagenesis has taken place, chemical cementation preserves the microstructure of the sediment and only minor changes in clay particle arrangements take place during further mechanical compaction, even when very high stresses are used. Also, it is seen that chemical processes reduce the permeability more strongly than mechanical compaction.