The known Re(V)=O complex of D-penicillamine (D-penH(4); the subscript indicates the number of dissociable protons) is isolated as a neutral form, ReO(D-penH(3))(D-penH(2)) (1). The complex is six-coordinate with one CO2- bound trans to the oxo ligand, resulting in a cis-N-2, cis-S-2, trans-O-2 geometry. One carboxyl group is anti and the other is syn to the oxo ligand; only the anti-pen CO2- can coordinate. For 1, in high-pH solutions, we have found unusual H-1 NMR spectral properties that have led to the discovery of a facile oxo ligand exchange process. 1 was also studied by acid-base titration, which revealed that the neutral-pH form (I) was [ReO(syn-D-penH(2)-N,S)(anti-D-penH(2)-N,S,O)](-). Significant spectroscopic changes occurred at pH similar to 11 and above. The two sets of pen H-1 NMR signals observed at neutral pH decreased in intensity as the pH was raised, and several original signals shifted.