The crystal-chemistry of a series of synthetic Al–Fe3 + smectites was studied in detail using near and mid infrared spectroscopy. Chemical and NIR data indicated a quite complete range of octahedral Al for Fe3 + substitution, and therefore, the solid-solution between beidellite and nontronite end-members was continuous and complete. The wavenumbers of several infrared absorption bands were correlated with the chemistry of the synthetic smectites, providing a useful tool to constrain their structural formulae and also for assisting in assignments of similar bands in natural smectites. The Al and Fe3 + cations were shown to be randomly distributed in the octahedral sheet of synthesized smectites. Despite the high availability of iron during synthesis, generally only a small amount of tetrahedral Fe3 + was observed.