We report the two-dimensional compressibility of electrochemically deposited lead on silver (111). Measurements were made in situ (in contact with solution) using grazing incidence x-ray scattering. Between monolayer formation and bulk deposition, the near-neighbor distance of the lead monolayer decreases linearly with applied potential. Since the lead monolayer is in equilibrium with the lead in solution, the isothermal compressibility of the monolayer can be calculated and is 0.98 2/eV. This is in good agreement with that of a two-dimensional, free-electron gas. © 1988 The American Physical Society.