Clay mineral analysis of Spodosols collected from the Adirondack Mountains reveals that smectite is common in the forest floor and uppermost soil horizons (the O, A and E horizons) and probably forms from the transformation of vermiculite via a low-charge vermiculite intermediate. The conversion of vermiculite to smectite occurs in the upper part of the soil profile where organic acids and strong inorganic acids (derived from atmospheric deposition) combine to create an intense weathering environment. X-ray diffraction (XRD) and chemical data for the clay fraction indicate that both the smectite and the low-charge vermiculite are Al-rich and dioctahedral. The smectite appears to be a beidellite. Transformation of vermiculite to smectite may have progressed in these acidic horizons by net layer-charge reduction resulting from the progressive substitution of Si for Al. The parent material for the soil clays was probably biotite, but little remains in these soils.