Chromium is mobile in ultramafic magmas but its mobility in high temperature fluids has long been unclear. Studies of some chromium-rich ophiolites have suggested chromium mobility in upper mantle fluids. However, the mechanism is poorly understood because Cr(III) is so insoluble in water. We used previous estimates of aqueous Cr-species and published experimental and ab initio studies of the solubility of 𝐶𝑟₂𝑂₃ in 𝐻𝐶𝑙 and 𝐾𝐶𝑙 fluids at 500 -1,200 °C and 0.1 -6.0 GPa together with the Deep Earth Water Model to calibrate a thermodynamic equation of state for the Cr(II) complex 𝐶𝑟𝐶𝑙(𝑂𝐻)⁰. Our model predicted low Cr solubilities (0.15 𝑚𝑔∙𝑘𝑔_H2O^-1) in a mid-ocean ridge fluid and very high solubilities (3,500 𝑚𝑔∙𝑘𝑔_H2O^-1) in saline, peridotitic diamond-forming fluid consistent with expectations for these environments. In pelitic, mafic eclogitic, and serpentinite fluids, predicted Cr solubilities varied widely depending on the oxygen fugacity and Cl concentration. The predicted predominance of Cr(II) in deep fluids and Cr(III) in minerals suggests that precipitation of Cr-mineral components is a redox reaction.