In situ scanning tunneling microscopy (STM) has been combined with infrared reflection-absorption spectroscopy (IRAS) to yield detailed atomic-level adlayer structures for saturated coverages of CO on ordered Rh(l11) in aqueous solutions. Two distinctly different structures were obtained in CO-containing 0.1 M NaClO4, depending on the electrode potential. At higher potentials, ca. −0.1 to +0.3 V vs SCE, atomic-resolution STM images were obtained that indicated the presence of a (2 × 2)-3C0 unit cell, having a CO coverage θCO = 0.75 in agreement with electrochemical and IRAS measurements. The corresponding in situ infrared spectra indicate the presence of two atop (or near-atop) and one 2-fold bridging CO in the unit cell. A real-space structure is suggested that is related to the corresponding (2 × 2)-3CO adlattice on Rh(l11) in ultrahigh vacuum as deduced previously by low-energy electron diffraction and vibrational spectroscopy. At potentials negative of −0.2 to −0.1 V vs SCE, markedly different STM images were obtained, having the symmetry (3 √3 rect)-4C0 (θCO = 0.67). This reversible potential-induced structural alteration as discerned by STM correlates with the substantial changes observed in the infrared spectra. The latter indicate the predominant presence of bridging CO in the (3 × √3 rect) structure, with only one CO per unit cell being coordinated at an atop site. Both the STM images and the IRRAS data suggest that two CO’s occupy “asymmetric bridging” positions. The increased preference for bridge bonding at lower potentials is consistent with the greater extent of dπ-2π metal-CO back-bonding expected under these conditions. The virtues of parallel STM and IRAS measurements for deducing ordered adlayer structures of such unprecedented atomic-level detail at metal-solution interfaces are emphasized.