Chromium K-edge X-ray absorption near-edge structure (XANES) spectra were recorded at room temperature for 27 CaO-MgO-Al2O3-SiO2 (CMAS) glass compositions quenched from melts equilibrated at various oxygen fugacities (fO2) at 1400 °C. Values of Cr 2+/∑Cr were determined from the intensity of a shoulder on the main absorption edge, attributed to the 1s → 4s transition, which is characteristic of Cr2+ in these glasses. For each composition, Cr2+/∑Cr could be quantified as a function of fO2, using a theoretical expression, from as few as three samples (Cr2+/∑Cr 0, 0.5, and 1). This allowed logK′, or the reduction potential of the Cr3+/2+ half-reaction, and hence the relative change in the ratio γmeltCr3+O1.5/γmeltCr2+O, to be determined for each composition. At constant fO2, log[Cr2+/Cr3+] was found to decrease linearly with increasing optical basicity. The variation in logK′ with composition is controlled by γmeltCr3+O1.5, corresponding to the capacity of the melt to stabilize both the charge and the preferred solvation site of Cr3+. The method was then applied to spectra recorded in situ at 1400 °C for a synthetic mid-ocean ridge basalt (MORB) composition, allowing Cr2+/∑Cr to be quantified in a Fe-bearing melt for the first time. Cr2+/∑Cr was found to vary from ~0.45 at the nickel-nickel oxide (NNO) fO2 buffer to ~0.90 at iron-wüstite (IW). This indicates that Cr2+ is likely to be the dominant oxidation state in terrestrial basaltic melts.