As a consequence of the strong preference of Ca 2+ ions for M2 sites in clinopyroxenes, the Fe 2+/Mg 2+ ratios in clinopyroxenes in equilibrium with a given orthopyroxene are very sensitive to Ca 2+ content. This places in doubt the use of the Fe 2+-Mg 2+ exchange equilibria between clino and orthopyroxenes as a 'thermometer' and means that the Ca 2+ content of clinopyroxenes must be accurately known for meaningful thermodynamic analyses. All available data on Fe 2+-Mg 2+ exchange equilibria between M1 and M2 sites in orthopyroxenes and between olivine and orthopyroxene phases have been reanalyzed and can be fit to the ideal two-site model of Mueller with only two free energy parameters. The standard free energy for exchange between M1 and M2 sites, ∆GE°, which is consistent with all the data from 900-1300°C is 3.6 kcal/gfw, the same value as was deduced by Virgo and Hafner from Mössbauer data for the range 600-1000°C. The free energy change, ∆GT°, for the reaction Fe2SiO4 + 2 MgSiO3 ⇄ Mg2SiO4 + 2 FeSiO3 is 1.65 kcal/gfw. From this, the standard free energy of formation of ferrosilite from wüstite and β-cristoballite deduced at 900°C is -1.8 kcal/gfw. The ideal two-site model is not consistent with measured activity coefficients in orthopyroxenes or in olivines indicating significant cancellation of activity coefficient ratios in the ion-exchange equilibria.