Expanding and non-expanding layers of interstratified clay minerals have been examined by high-resolution transmission electron microscopy. Permanent expansion of swelling layers under the electron beam was achieved by intercalation of n-alkylammonium ions, especially the octadecylammonium ion. Oriented flakes of clay minerals were prepared by embedding the expanded or non-expanded clay minerals in epoxy resin, followed by centrifugation before hardening of the resin. The minerals were then cut perpendicular to 001 using an ultramicrotome. Crystals of macroscopic trioctahedral vermiculites show homogeneous interplanar distances of 24 Å after intercalation of octadecylammonium ions. Crystals of dioctahedral soil vermiculites often show a central zone with non-expanding 10 Å layers; the outer zone shows a disturbed layer sequence extensively expanded by n-alkylammonium ions. After embedding in epoxy resin, vermiculites show stable 9·2 Å interplanar spacings but smectites expand to 13 Å. Montmorillonites of the Wyoming type show curved stacks of layers. Most of the layer stacks of montmorillonites of the Cheto type are split and disordered aggregates of single layers are formed. Crystals of illites and glauconites are built up of aggregated small stacks of 10 Å layers, the layer stacks consisting of 10 layers. Mostly the boundaries of the layer stacks are parallel to their 001 planes; sometimes low-angle boundaries are found. The dimensions of the layer stacks, ∼ 100 Å thick and some hundreds of Å in plane, are equal to the dimensions of the domains of coherent scattering of X-rays. The border layers between the layer stacks are identical with those 5 to 10% of layers which swell with glycerol or ethylene glycol during X-ray analysis. Some of the layer stacks of illite and glauconite crystals are expanded by octadecylammonium ions within a fortnight. The other stacks show unchanged 10 Å spacings. The different expanding behaviour of different layer stacks reflects the heterogeneity of the layer-charge distribution in the mica clay minerals. K-bentonites show the same expanding behaviour as illites and glauconites but the number of layers expanding with octadecylammonium ions is greater in K-bentonites than in illite crystals. Expanded mixed-layered minerals of the illite-smectite type show a different layer stacking sequence from illites. Random irregular stacking of mica layers with expanded layers are recognized rather than coherent stacks of mica layers. The crystals have a stepped morphology, perhaps effected by translations along the 001 plane. After reaction of the rectorite from Garland County with octadecylammonium ions, the non-expanded mica layers and the expanded smectite-like layers can be distinguished. The heterogeneity of the interlayer charges of the smectite layers is documented by the formation of alkyl double-layers with 17 Å spacings and alkyl triple-layers with 21 Å spaces in irregular sequence. The ‘rectorite’ from the Goto Mine expands nearly homogeneously in comparison with the rectorite from Garland County. After reaction with octadecylammonium ions, interplanar spacings of mostly 31 Å are observed but rarely spacings of 27 Å. The smectite layers of the corrensite from Kaubenheim are expanded by tetradecyl-ammonium ions to 18 Å spacings by formation of alkyl double-layers. A regular 1 : 1 layer structure of 14 Å chlorite layers and expanded 18 Å smectite layers with total spacing of 32 Å can be observed. Muscovite and pyrophyllite are not expandable by n-alkyl-ammonium ions within a fortnight. However, sporadic layers of celadonite crystals are expanded. Generally the 10 Å or 9·2 Å layers extend over the whole crystals of the three minerals. In celadonite crystals, disorder is caused sporadically by interrupted layers or slightly enlarged layer spacings.