We studied the oxidation state of Fe in silicate glasses produced during the first atomic bomb blast at the Trinity test site (New Mexico) by X-ray absorption–near edge spectroscopy (XANES). The sample consists of green glass resulting from melting of the quartz-bearing sand present at the test site; some relict unmelted sand is still fused to the bottom of the sample. Comparison of the pre-edge peak data with model compounds of known Fe oxidation state and coordination number shows that in the Trinity glass sample, Fe is in the divalent state and, on average, in a mixture of 4- and 5-fold coordination. XANES spectra collected at various heights of the sample, from the bottom of the sample up to the exposed surface, show no variation of the pre-edge peak and, thus, of the Fe oxidation state with the distance from the sand-glass interface. However, XANES analysis of a portion of the sand at the bottom of the sample shows Fe to be a mixture of Fe 2+ and Fe 3+ , with a Fe 3+ /(Fe 2+ + Fe 3+ ) ratio close to 0.5. This demonstrates that during the nuclear explosion, the ground rock was instantaneously reduced, transforming all the iron from mostly trivalent state to almost exclusively divalent. Pre-edge peak features (intensity and energy) are consistent with those of tektites from the Ivory Coast studied here and with literature data of tektites from all the other known strewn fields (Australasian, Central European, and North American). The reduction of Fe to divalent state during Trinity glass formation, the homogeneity of the Fe oxidation state within the glass, and the Fe structural role suggest that this glass represents a good analog of tektite glass.