The weathering of feldspars and micas begins with a series of surface reactions in which metallic cations of the mineral (e.g. Na, K, Ca, Mg) pass into solution in exchange for H ions or other cations. However, many of our simple concepts of exchange reactions fail. For instance, the exchange capacity of feldspar surfaces is not a constant but varies greatly with the exchanging cation. The ammonium ion penetrates the surface readily and gives a high capacity; Mg gives a much lower value. Ammonium also becomes fixed at feldspar surfaces and can then only be removed by boiling in alkali. In all exchange reactions with feldspar surfaces, the previous history was shown to be of great importance. These facts are explained by the formation of a surface layer consisting of somewhat loosened and distorted feldspar chains. The micas, on the other hand, showed much more normal surface exchange. However, the geometry of the exchanging sites was found to be of lay great importance. These were shown to be of more than one kind, characterized by different bonding energies towards cations. Further, the exchangeability of any one cation was highly dependent upon the extent and kind of occupancy of the exchange sites. This was very strikingly shown by curves in which the selectivity numbers were plotted against surface occupancy. These reactions represent the first step in weathering processes, at a stage when new mineral species have not yet made their appearance.