The layer structure and chemical composition of individual packets of 2:1 layer silicates in illitic materials from various environments have been studied with analytical high-resolution electron microscopy using ultrathin sections treated with n-alkylammonium. Individual packets consist of a polar or nonpolar structure, as judged by the response to treatment with n-alkylammonium. The status of illite-smectite mixed-layers attributed to these materials is found to reflect the composition and arrangement of polar and nonpolar 2:1 layer silicates. The nature of the 2:1 mixed-layer silicate thus cannot be ascribed to a mixture of illite and smectite layers. In our opinion, a definition based on TEM reflects the true nature of mixed-layer materials more accurately than one based on XRD: it leads us (1) to distinguish among different types of expandable and nonexpandable components; (2) to document variations in the density of interlayer charge of the expandable components, which are not detectable by XRD; (3) to distinguish between expandable and nonexpandable illite, which do not respond differently to ethylene glycol treatment; (4) to distinguish between fundamental particles and short-range ordered structures; (5) to relate the ordering of the T-O-T layers to the chemical composition, as measured directly by TEM (XRD study of oriented samples gives information on the interlayer spacing only); and (6) to characteize all types of particles present (not only the coherent sequences, as given by XRD).