Abstract 57Fe mössbauer spectroscopy has been used to examine some naturally occurring layer silicates in which cations located in exchange sites in the interlayer regions can be replaced by other species. The 57Fe Mössbauer spectra recorded from differing size fractions of two types of non-exchanged and sodium-exchanged montmorillonite clays were found to be independent of the fraction size. The spectra have been interpreted in terms of the occupation by iron(III) of a heterogeneity of similar sites within the montmorillonite lattice. No justification has been obtained for computer analysis of the data in terms of more than one characterisable lattice site and no evidence has been found for the association of any iron oxyhydroxide impurity with the montmorillonite fractions. The 57Fe Mössbauer parameters recorded from iron(III)-exchanged montmorillonite, in which iron(III) species are intercalated within the layers, show that the process is best performed at fairly low pH using low concentrations of iron(III). Failure to control such conditions can result in the formation of iron oxyhydroxides or hydrolysed iron(III) species. The preparation of iron(II)-exchanged montmorillonite was accompanied by partial oxidation of the iron(II) to iron(III).