By adding antimony the system MnBi is extended to a three‐component one. Using Sb concentrations not exceeding a critical value one gets facilitated conditions for preparation of magneto‐optic thin films of the B8 structure with preferred orientations of the c‐axis. Thereby a number of advantages could be obtained, compared with “pure” manganese bismuth films, e.g. relatively low formation temperatures and short formation times (in the order of 200 °C and 1 h, respectively), excellent film homogeneity, use of amorphous substrates instead of epitaxial materials. Films with surface protective layers of good quality show no significant difference whether the thermal formation of the vacuum‐deposited three‐layer‐systems BiSbMn takes place in vacuum or in air. The effect of antimony additions to manganese bismuth has been demonstrated by observations chiefly obtained in a heat stage microscope. The results of these investigations are reported.