High pressure‐temperature aqueous fluids are essential to melt generation, element cycling, and fluid‐melt‐rock reactions occurring in subduction zones. Recent advances in theoretical thermodynamic modeling help facilitate calculations at a range of pressure conditions relevant to subduction zones. Here we explore stable and metastable equilibrium speciation of C1 and C2 aqueous carbon species along a theoretical slab surface pressure‐temperature path. These calculations reveal a thermodynamic drive to stabilize small organic compounds at elevated pressures and temperatures, with pH buffered by the diopside‐antigorite‐forsterite mineral assemblage. At stable equilibrium, oxidized forms of aqueous carbon dominate the speciation at and above oxidation conditions set by the fayalite-magnetite‐quartz (FMQ) assemblage. Under conditions more reduced than FMQ, a larger variety of aqueous carbon species are stabilized. If metastability were to persist along the path targeted by this study, it is predicted that a plethora of C1 and C2 aqueous species would be stabilized, especially under reduced conditions. These results point the way for theoretical geochemical modeling in the pressure‐temperature‐composition space of sub-duction zone fluids and provide new constraints on forms of deep carbon.