The stability relations of prehnite and epidote were experimentally determined for the bulk composition of Ca2Al2FeSi3O12(0H) + excess H2O using conventional hydrothermal apparatus and solid oxygen buffer techniques. Three starting materials were used: (I) mixtures of natural prehnite and hematite, (II) mixtures of synthetic epidote and natural prehnite + hematite in subequal proportion; and (III) reground run products. Over a range of ∫02, epidote was readily synthesized from starting material (I) at 500–550 °C, 5–8 kb Pfluid, run duration of 7 to 45 days, and prehnite grew at the expense of its high-T epidote assemblage at temperatures below 350 °C, 2–8 kb, run duration of 20 to 50 days. Grandite garnet was in some cases detected, especially in the synthetic run products at lower ∫02 values and at higher temperature in longer experiments. Microprobe analyses indicate that the Fe3+/(Fe3+ + Al) ratios of synthetic epidotes and prehnites vary as a function of ∫02, and temperature. Both epidote and prehnite are most iron-rich at 325 °C and 2 kb, 353 °C and 5 kb and 373 °C and 8 kb and ∫02, of the HM buffer; they become more aluminous with decreasing ∫02, and with increasing temperature. Such change in composition is consistent with their variations in cell dimensions. The phase relations of prehnite and epidote are defined by continuous reactions where a transition zone between the garnet-in reaction and the prehnite-out reaction occurs at 325–343 °C, 2 kb, 353–375 °C, 5 kb and 373–395 °C, 8 kb Pfluid and ∫02 of the HM buffer. The minimum stability for the garnet-bearing assemblages is defined by the garnet-in reaction, and the maximum stability for the prehnite-bearing assemblages by the prehnite-out reaction. Sliding equilibria involving phases with continuous change in composition apparently are common for low-grade metamorphism. The present results combined with previously determined relations for prehnite, zoisite, and epidote delineate a petrogenetic grid which is consistent with parageneses and variation in Fe/Al ratios of coexisting prehnite and epidote in metabasites of the prehnite–pumpellyite and pumpellyite–actinolite facies and in altered rocks from many geothermal systems.