An investigation was made to det. whether and under which circumstances Co could play a role analogous to that of Fe in ferrites. In the theoretical part, certain "rules" were derived to make a comparison on the basis of a close-packed anionic structure, and to approximate the Curie (or Neel) temp. In the exptl. part, no anhyd. compds. of the Co(II) oxyfluorides could be found. Several Co(II) hydroxyfluorides and a fluoride hydrate were obtained for the first time. Attempts to replace Fe+++ by Co++ and other high-valence cations in KFe11O17 and related β-Al2O3 compds. were unsuccessful. A large proportion of Ga+++ could be introduced in KFe11O17, producing a magnetic behavior which possibly indicates a change in coupling between the spinel blocks of the structure.