Three sulfur-buffering, sulfide-silicate reactions, which limit the stabilities of the skarn silicates andradite garnet and hedenbergite pyroxene under sulfidizing conditions, were studied hydrothermally at a pressure of 2 kb over the temperature range 700 degrees to 350 degrees C:Reaction (1): andradite + S 2 = wollastonite + anhydrite + quartz + pyrite or pyrrhotite,Reaction (2): andradite + magnetite + S 2 = anhydrite + quartz + pyrite or pyrrhotite, andReaction (3): hedenbergite + S 2 = andradite + quartz + pyrrhotite.The equilibrium value of the sulfur fugacity (f (sub S 2 ) ) for each reaction assemblage at any given temperature was calculated from the measured composition of a coexisting sulfur fugacity indicator (pyrrhotite or sphalerite, as appropriate). The log f (sub S 2 ) ) data were adequate for the determination of the log f (sub S 2 ) )-1/T relations for each of the andradite sulfidation reactions. Owing to excessive scatter in the data for the hedenbergite sulfidation reaction, however, the log f (sub S 2 ) -1/T relation for this reaction, derived from the log f (sub O 2 ) data of Burton et al. (1982), is preferred to the one developed by Gamble (1976, 1978). The log f (sub S 2 ) -1/T relations for the three sulfidation reactions were combined with equilibrium data available in the literature for other reactions in the model system Ca-Si-Fe-O-S and used to construct quantitative, isothermal-isobaric, log f (sub S 2 ) -log f (sub O 2 ) phase diagrams applicable to the formation of Ca-Si-Fe skarns. The quantitatively derived phase relations were found to be consistent in all major respects with the schematic phase relations deduced by Burt (1972a and b).The influence of diopside component on the stability of hedenbergitic pyroxene was also studied. Diopside was found to enlarge the stability field of hedenbergitic pyroxene, increasing the temperature at which pyroxene becomes stable with pyrite from 288 degrees C for end-member hedenbergite (Burton et al., 1982) to 395 degrees C for 50 mole percent diopside. Diopside was found to be less effective than the johannsenite component in stabilizing hedenbergitic pyroxene.