We have applied scanning tunneling microscopy (STM) and atomic force microscopy (AFM) to study particle shape and thickness in illite/smectite (I/S) from the North Sea Jurassic oil source rocks. We demonstrate that STM can be used on our I/S particles, despite their poor conductivity, if they are ultrathin (~10 Å) and, furthermore, that detailed surface topography of our I/S particles can be mapped by STM and AFM and even at the atomic resolution level by AFM. The accurate thickness measurements show that our material contains lath-shaped particles l0 Å thick, which are elementary smectite particles according to the fundamental particle concept but can be both smectite and mica (illite) according to the AIPEA classification. Surface adsorbed layers, probably of H2O, can be seen in STM images of I/S particles that previously were cleaned for organic compounds and iron and aluminum oxides. On I/S particles, STM and AFM give data that hitherto have not been obtained by other methods and that provide insight into clay mineral diagenetic processes and into adsorption on clay mineral surfaces.