Mineralogical trends have been investigated on a detailed scale in two Westphalian fluvial sandstone sequences in contact with either a coal seam or shales. The evolution in an authigenic clay mineral assemblage can be related to changes in the pH of the pore-water. Firstly, kaolinite formed early in diagenesis, mainly as a result of K-feldspar dissolution and alteration. This process, which took place under acidic conditions, consumed protons. Subsequently the pH of the pore-water increased and after compaction, illitization of kaolinite occurred under near neutral conditions. Deep burial is marked by dickite formation, which again reflects acidic conditions. Distribution of clay minerals is related to meteoric water flux through the sediments and the release of organic acids and CO2 from coalification of organic matter in the underlying strata.