Despite the current availability of several crystal structures of purple acid phosphatases, to date there is no direct evidence for solvent-derived ligands occupying terminal positions in the active enzyme. This is of central importance, because catalysis has been shown to proceed through the direct attack on a metal-bound phosphate ester by a metal-activated solvent-derived moiety, which has been proposed to be either (i) a hydroxide ligand terminally bound to the ferric center or (ii) a bridging hydroxide. In this work we Use H-2 Q-band (35 GHz) pulsed electron-nuclear double resonance (ENDOR) spectroscopy to identify solvent molecules coordinated to the active mixed-valence (Fe3+Fe2+) form of the dimetal center of uteroferrin (Uf), as well as to its complexes with the anions MoO4, AsO4, and PO4. The solvent-derived coordination of the dinuclear center of Uf as deduced from ENDOR data includes a bridging hydroxide and a terminal water/hydroxide bound to Fe2+ but no terminal water/hydroxide bound to Fe3+. The terminal water is lost upon anion binding while the hydroxyl bridge remains. These results are not compatible with a hydrolysis mechanism involving a terminal Fe3+-bound nucleophile, but they are consistent with a mechanism that relies on the bridging hydroxide as the nucleophile.