Summary: A muscovite that contains iron almost entirely in the ferric form has been studied by Mössbauer spectroscopy. It is established that Fe³⁺ ions in the sites with cis hydroxyl groups have similar parameters in both muscovite and biotite. By assuming that Fe²⁺ ions will also have similar parameters for these sites in both minerals the Mössbauer results of other workers have been reinterpreted. This new interpretation, which is in complete agreement with the X-ray structural analysis of Radoslovich (1960), indicates that in muscovite the sites with cis hydroxyl groups are filled in preference to those with trans hydroxyls. These sites are occupied primarily by trivalent ions. There is possibly a preference for divalent ions by the unique sites with trans hydroxyl groups, but occupation of these sites appears to occur only when the other sites have been filled.