We present an x-ray-diffraction structural analysis of the b-Ge(111)-(Ö3 × Ö3)R30°-Pb ® 1 × 1 phase transition at ~ 180 °C for a Pb coverage of 1.25 ML. We have studied the atomic structure below and above the phase transition by measuring the distribution of diffracted intensities along integer-order rods of Bragg scattering. Below the phase transition, the b phase has a saturation coverage of 4/3 ML. We find that above the phase transition the single layer of Pb gives rise to a ring of diffuse scattering indicative of a two-dimensional liquid. However, of all the Pb geometries considered, an ordered layer with high in-plane thermal vibration amplitude is found to provide the best agreement between calculated and measured structure factors. The Pb layer thus has both liquid and solid properties.