Processes that may be important during the weathering of clay minerals in soils or other geologic materials occur under laboratory conditions. These are: 1) the movement of Al ions from clay lattice positions to ion-exchange sites; 2) the hydrolysis of Al ions in clay systems with the deposition of Al(OH) 3 , or the sorption of hydroxy-aluminum ions. The first reaction proceeds under conditions of acid weathering when metal ions such as Ca, Mg, or K are displaced by H ions. Its rate is much accelerated by increase in temperature. The hydrolysis of Al ions, which may occur either in solution or on change sites, produces H ions that can cause further clay decomposition. It is suggested that the 2 processes operating together can provide a reaction mechanism that removes Al from coordination positions in clay mineral crystals and deposits it as a hydrous oxide. Under conditions where silica is removed by leaching or crystallization as secondary quartz such a reaction could lead to the conversion of alumino-silicate clay into aluminum oxides.