Abstract: The occurrence of mixed-layer clays was noticed in several New England soils; a particularly well developed mixed-layer sequence involving the weathering of randomly interstratified vermiculite and illite was intensively studied. Weathering of the interstratified system resulted in the formation of alumina in interlayer positions as evidenced by chemical and x-ray tests. The change in “apparent” spacing of the clay was found to be statistically correlated with sample depth which is in turn related to the intensity of weathering. The change in “vermiculite” spacing from 14.2 to 13.9 A was also found to be correlated with depth. The diffraction effects of the interstratified mineral are discussed in the light of the Hendricks and Teller formula and Méring's refinement of the formula. The evidence points to the alumina being “brucitic” in character; this modifies the curves calculated by Brown and MacEwan for the randomly interstratified 10 to 14 A mineral. The role of interstratification in the process of soil formation is discussed.