A complex coverage- and temperature-dependent LEED pattern has been observed for water adsorbed on clean Ru(001). This pattern consists of the (1×1)-Ru spots, ( 7/8 × 7/8 ) R 30°-H2O spots and a water-induced halo around each integral order spot at low temperatures (80 to 150 K). At higher temperatures, the integral halo split into a distinct hexagon of satellite spots oriented in the 7/8 directions and the 7/8 spots formed streaks in a threefold symmetric pattern. The radius of the integral halo (hexagon) increased continuously with increasing water coverage to a maximum 20%–25% of the radius of the 7/8 beam above three bilayers coverage. This pattern has been modeled in terms of an antiphase domain structure of two-dimensional, hydrogen-bonded bilayer domains. A computer simulation of the LEED patterns arising from this model structure gives good agreement with the experimental results. Other considerations in the model determination were based on simple water bilayer island models which were developed to explain experimental thermal desorption and electron stimulated desorption ion angular distribution data.