Variable-temperature X-ray diffractometry, infrared spectroscopy and multinuclear conventional and magic-angle-spinning n.m.r. (²⁹Si, ²⁷Al, ¹³C, ¹H) spectroscopy have been used to (i) identify the location of isomorphous elements (e.g. Al in 4- or 6-coordinated sites) in typical and model clay catalysts, (ii) clarify the nature, disposition and mobility of interlamellar species (both organic and inorganic) and (iii) pinpoint significant structural differences, especially following heat-treatments, of a range of cation (Li⁺, Na⁺, Cs⁺, NH⁺₄ and Al³⁺)-exchanged montmorillonites. In particular we show that: (a) Montmorillonite samples containing either relatively small interlamellar cations (such as Li⁺) or cations of the type NH⁺₄ or Al³⁺, which ‘release’ protons at high temperatures, show a change in the ²⁹Si chemical shift from –93.5 to –95.5 ppm on heating to 500 °C, this being the consequence of diffusion of the Li⁺ or H⁺ into the sheets with ensuing decrease of layer charge. (b) Cation diffusion into the sheets is also reflected by the decrease in intensity of the i.r. absorptions at 913 and 844 cm^–1. (c) Structural dehydroxylation at 700 °C is accompanied by the disappearance of octahedrally coordinated aluminium and the formation of 5-coordinate aluminium, as seen in the ²⁷Al magic-angle n.m.r. signal. (d) Dipolar-coupled structural hydroxyls have a ¹H chemical shift of 1.2 ppm, as measured by magic-angle-spinning n.m.r. spectroscopy.