The magnetic anisotropy of ultrathin Co films on Au (111) changes with increasing film thickness in the regime of 3–5 monolayers (ML). This leads to a reorientation of the magnetization from the out-of-plane to the in-plane direction. We observed this transition by means of magnetic-force microscopy under UHV conditions. We found that by approaching a film thickness of 4 ML the average domain size shrinks from about (Formula presented) to 400 nm. For films thicker than 4 ML we have observed an in-plane domain structure. The magnetic structure of the ultrathin Co films was found to depend additionally on the chemical composition. For a Co film of a thickness just above the critical thickness for the reorientation transition the magnetic structure reverted from in-plane to out-of-plane due to carbon contamination of the thin films. We also found that the critical Co film thickness for the reorientation transition could be shifted even up to 18 ML by the presence of carbon.