The significance of a single CO2-H2O fluid phase is well known for metamorphic systems, and CO2-H2O immiscibility is explicit in fluid inclusion literature, especially regarding hydrothermal ores. Complex multiphase CO2-H2O behavior exists over wide temperature and pressure ranges overlapping other important geochemical processes. The character and physical-chemical properties of multiple phases possible for CO2 and H2O, and the potential impact of these coexisting phases on geochemical processes in the crust, are not broadly appreciated. We propose that immiscible supercritical CO2 fluid and a liquid rich in H2O coexist in the shallow crust, to 400¿C and 300 MPa, and that interactions among the two fluids and host rock are significant processes that produce recognizable geochemical and textural evidence. Supercritical CO2 fluids bring potential complexity to fluid-rock systems by influencing aqueous reactions via carbonic acid equilibria, penetrating complex geometries inaccessible to aqueous fluid, and dissolving and redistributing metals as organometallic compounds. The distal margin of a contact metamorphic aureole is one example we discuss in which interaction between two disparate CO2-H2O fluids controls H2O activity and the progress and distribution of metamorphic hydration reactions. In another example, supercritical CO2 produces acidity, carbonate saturation, and silica supersaturation in brine. Separation and emplacement of this brine into a rock-dominated system buffered to neutral pH enhances precipitation of carbonates and quartz, chalcedony, or amorphous silica in veins. Other possible examples of CO2-H2O fluid immiscibility coupled with multiphase fluid-rock interactions are clay desiccation, diagenetic and postdiagenetic silicate reactions, origin and distribution of carbonate cements in sedimentary basin sandstones, fluid-mass transfer, and anthropogenic CO2 sequestration.