Antiferromagnetic domains at surfaces of NiO single crystals were studied by polarization dependent soft X-ray absorption at the Ni2p absorption edges. Spatially resolved information is obtained by monitoring the absorption via the total yield of electrons in a photoemission electron microscope. On the (100) surface, stripe shaped domains are found with widths of the order of a few 10 μm, extending over 100s of μm. The polarization dependence shows a sign change of the contrast for every 90° of rotation, as expected for the antiferromagnetic twin domains with the major portion of the antiferromagnetic axis within the plane of the surface. The domains are not affected by surface topography. This suggests that the formation of domains is governed by defects in the bulk, which lead to nucleation of domains at specific sites.