Vermiculitization of mica is one of the main weathering processes in soils from three upland catchments receiving various levels of acid deposition. This process is manifested by the presence of interstratified mica-vermiculite with the interlayer space in the vermiculite partially filled with polymeric hydroxyaluminium species. In one peaty podzol, regularly interstratified mica-vermiculite developed at the expense of mica and was the dominant mineral in the Eh horizon. It was concentrated by chemical treatments to remove organic matter, free iron oxides, and many Al species in the interlayer, and the structural formula calculated from chemical analysis confirmed the dioctahedral character of both vermiculite and mica components, and indicated that the vermiculite weathering product was formed from a dioctahedral mica. The degree of interlayering in the interstratified phases seemed to be pH-dependent with resultant implications for soil and freshwater acidification.