Hydroxy-Al and -Fe interlayers were prepared in "mono-mineralic" fine and coarse clay montmorillonite and coarse clay and silt vermiculite at 3~ and 21~ The formation of hydroxy interlayers was evaluated by X-ray diffraction, cation exchange capacity, and chemical analyses. At comparable particle size and regardless of temperature, the amounts of hydroxy-Al and -Fe interlayers in montmorillonite exceeded by far those formed in vermiculite. Likewise, the aluminum systems exhibited a higher degree of interlayering than iron systems. Within montmorillonite, the amount of hydroxy-A1 and -Fe interlayers increased as the particle size decreased, regardless of temperature. The aluminum interlayered montmorillonite equilibrated at 3~ was characterized by basal spacings of about 17 ,~ after Ca-saturation plus 54 per cent relative humidity. These spacings are larger than those normally observed for smectites. Within vermiculite systems equilibrated at 3~ more hydroxy-AI interlayers were recorded in coarse clay than in silt fraction, whereas at 2 I~ about equal amounts of interlayers were formed. By contrast, hydroxy-Fe interlayer was favored by the silt fraction at both temperature levels. The formation of aluminum interlayers in both minerals increased with increasing temperature. The formation of hydroxy-Fe interlayers in montmorillonite was generally not temperature dependent, whereas the formation of such interlayers in vermiculite increased slightly with increasing temperature. These data may partially explain the formation of chloritic intergrades in soils as a function of type of minerals, kind of ions, and thermal variations.