Aluminous hydroxy 2/1 clay minerals (‘intergrades’) can form in two ways: Al can either come externally from the solution, or internally from the mineral itself (octahedral or tetrahedral layers). This study is more concerned with the latter which seems the main source in acid soils.The most common organic acids present under natural conditions were used on two trioctahedral micas (phlogopite and biotite), and their chemical and mineralogical weathering effects were determined.Different kinds of mineralogical evolution are possible: mineral destruction by a few very complexing acids, possibly with intermediate stages (interstratified minerals); formation of ‘transformation’ smectite by acids derived from sugars; Al-hydroxy vermiculite formation (the 14 Å diffraction peak of which is stable under K saturation and vacuum). This evolution is the most frequent and is produced by the most numerous organic acids (even CO2), the acidity of which results in Al migration in the interlayer, but the complexing ability of which is not sufficient for Al removal.