Summary: Well-resolved Mössbauer spectra of several dioctahedral and trioctahedral micas have been obtained and subjected to detailed computer analysis. Most of the spectra can be resolved into two ferrous quadrupole doublets and one ferric quadrupole doublet. In dioctahedral micas, ferrous iron is seen to occupy the larger, more symmetric octahedral site in preference to the smaller, less symmetric site, confirming predictions made on the basis of structure alone. In trioctahedral micas there is still a tendency for ferrous iron to occupy preferentially the more symmetric octahedral site even though the two octahedral sites are no longer distinguished by size. A lithium-rich biotite gives a spectrum typical of that expected from a mica with a zinnwaldite structure, but the spectrum of a more authenticated zinnwaldite could not be resolved sufficiently for detailed structural interpretation. The spectra of pyrophyllite and talc give results that can be related to the corresponding micas by considering their respective structural differences. In none of the micas studied was there evidence of ferric iron in tetrahedral coordination. The ferrous: ferric ratios obtained from the Mössbauer spectra do not always agree with the chemical values. In several cases the spectra show more ferrous iron than found chemically. It is suggested that the spectral values are more accurate, oxidation on chemical analysis being the most likely source of error.