The zeolite facies, formed under conditions intermediate between diagenesis and regional metamorphism, is characterized by mineral assemblages bearing laumontite, heulandite, or analcite, together with quartz. If H 2 O and CO 2 are regarded as mobile components, these assemblages, under the same conditions, rule out the following: calcite-kaolinite-quartz, calcite-pyrophyllite-quartz, or albite-quartz; these assemblages are all familiar in sediments, sedimentary rocks, or low-grade regional metamorphic rocks. The relations among these alternative assemblages are analyzed by means of isothermal, isobaric diagrams using as variables the chemical potentials of the 2 mobile components, H 2 O, and CO 2 . For a given reaction, it is not only possible to arrange correctly the relative dispositions of the pseudo-univariant around pseudo-invariant points, but also to assign numerical values to the slopes of the pseudo-univariant lines on the basis of the stoichiometric coefficients of the 2 mobile components. Such diagrams show that mineral assemblages of the zeolite facies can be obtained, isothermally and isobarically, by increasing the chemical potential of H 2 O relative to that of CO 2 . Thus the zeolite facies is not a necessary intermediate step between diagenesis and regional metamorphism.