The Rotliegend sandstones of northern West Germany are characterised by severe diagenesis with growth (in approximate time sequence) of: (1) iron oxide, (2) tourmaline, (3) authigenic feldspar (mainly albite), (4) illite, (5) quartz, (6) chlorite, (7) kaolinite, (8) anhydrite and carbonate. The illite forms (a) tangential rims, (b) radial rims, (c) mesh-work, (d) dense homogeneous masses. The chemical conditions responsible for this diagenesis involved alkaline pore fluids with high K+ and Na+. These conditions were initiated very early, sometimes only a few centimetres below the surface. The diagenetic materials came from detrital volcanic grains, and the key to extent of diagenesis may be in the amoung of dissolution of these grains.