We report in situ surface X-ray scattering measurements of the structure and compressibility of electrochemically deposited Bi monolayers on Ag(111) and compare the in situ structure with results from previous ex situ experiments on emersed electrodes, where the structure was studied with low-energy electron diffraction (LEED). We find that the Bi monolayer forms an unusual structure: a two-dimensional rectangular lattice that is uniaxially commensurate with the hexagonal Ag(111) surface along the Ag  direction. There are two Bi adatoms per rectangular unit cell and one adatom is displaced from the centered-rectangular position by ≃0.25 Å along the commensurate direction. The displacement shortens two of the Bi-Bi near-neighbor bond distances but lengthens two others and may reflect the tendency toward covalent bonding in Bi. With increasing coverage (decreasing electrode potential), the Bi monolayer compresses uniaxially along the incommensurate direction (Ag ), which preserves the uniaxially commensurate structure. The measured two-dimensional compressibility, ions does not affect the structure or its compressibility. Although the in situ rectangular structure differs from that proposed in the ex situ LEED experiments of Laguren-Davidson et al., we believe that the in situ and ex situ structures are in fact the same. We show that our proposed rectangular structure completely explains the LEED data, when multiple scattering (typically present in LEED) is taken into account; in contrast, the complicated (2√3×2√3)R30° structure proposed from LEED is inconsistent with our in situ data. This comparison demonstrates that, at least for the case of Bi/Ag(111), the ex situ emersion experiments preserve the in situ monolayer structure. © 1991, American Chemical Society. All rights reserved.