We report CW and pulsed Q-band H-1,H-2 ENDOR measurements of intermediate X formed during the assembly of the diferric tyrosyl radical cofactor of the R2 subunit in ribonucleotide reductase. These studies, performed with H2O and D2O buffers, were designed to determine whether the exchangeable proton signals are associated with an hydroxo bridge, a terminal water, or both. In doing so, we identify the types of protonated oxygen (OHx) species coordinated to the iron ions of X and their disposition relative to the ferric and ferryl iron ions. The exchangeable proton signals displayed by intermediate X belong to two protons associated with a terminal water bound to Fe,(III) and not to an hydroxo bridge; within the precision of the modeling, this picture of a terminal water is indistinguishable from that of a 2-fold disordered terminal hydroxyl. The fact that X displays strong spin-coupling between iron ions requires that there be one or more oxo/hydroxo bridges. These findings then establish that X contains the [(HxO)(FeOFeIV)-O-III] fragment.