Chemical data for 139 natural paragonite-muscovite (Pg-Ms) pairs illustrate the effects of ferromagnesian components on the P-T-X topology of the Pg-Ms solvus. The pairs were selected on the basis of: reasonably accurate knowledge of the P-T conditions of formation; evidence for close approach to equilibrium at peak metamorphic conditions; exclusion of pairs in which paragonite contains more than 5 mol% margarite; and exclusion of pairs from polymetamorphic rocks that contain more than one set of cogenetic Pg-Ms pairs. Graphical analysis reveals considerable scatter in the data; nevertheless, it is evident that the muscovite limb of the solvus shifts markedly toward end-member muscovite with increasing pressure from approximately 7 kbar to 21 kbar. This shift is attributed to a pressure-induced increase of the ferromagnesian content of muscovite, which increases the size of the XII alkali site - to the effect that K is more readily accommodated than Na. The data also suggest that the paragonite limb of the solvus migrates slightly toward end-member paragonite with increasing pressure. Broadening of the Pg-Ms solvus with increasing pressure reflects increasingly nonideal Na-K mixing as the phengite content of muscovite increases. Due to the wide scatter of data for Pg-phengitic-Ms pairs, it is concluded that, at the present time, Pg-Ms solvus thermometry is only viable for quasibinary Pg-Ms pairs.