The magnetic susceptibilities of polycrystalline MnCO3and MnO2have been measured in fields up to 300 kOe in the temperature region 80°–300 K. MnCO3shows an increase in susceptibility at about 190 kOe at 300°K. The low‐ and high‐field phases obey an antiferromagnetic Curie‐Weiss law, and constants &thgr;=61°±9°K, and &thgr;=44°±9°K respectively, MnO2shows a decrease in susceptibility at about 120 kOe and a subsequent increase at 270 kOe. The low‐, intermediate‐, and high‐field phases again follow an antiferromagnetic Curie‐Weiss law with constants &thgr;=266°±57°K, 110°±18°K, and 159°±12°K. The transitions are discussed in terms of a suppression mechanism by which the high fields affect the ligand orbitals, and estimates of the transition fields are made on the basis of this theoretical model.