A LEED analysis of the clean polar surfaces of CdS prepared by ion bombardment−thermal anneal technique reveals: (1) Only integral−order beams and six−fold symmetry from the (0001) −Cd face, and integral−order beams, as well as very diffuse √3×√3−30 deg reconstruction and mild facet formation, from the (0001̄) −S face; (2) Primary Bragg diffraction peaks can be extracted from LEED intensity−profile data via a structure factor analysis, which in turn can be used to obtain the inner potential (14.7 V), the lattice constant in the surface plane (4.10 Å), and the spacing between the CdS dipole layers normal to the surface plane (6.68 Å). The nominal bulk values for these dimensions are 4.14 and 6.71 Å, respectively. Exposure to oxygen (1 Torr−sec) has only a slight transient effect on the Cd face, even after subsequent annealing. Such exposure strongly reduces the LEED intensities of the S face, and subsequent annealing to 400°C produces macroscopic facets that can be detected by LEED and by transmission electron microscopy.