The ferromagnetic properties of manganese antimony (MnSb), in contrast to the corresponding compounds of arsenic and bismuth, do not show instabilities associated with first-order phase transitions near the magnetic ordering temperature. However, the magnetic properties of the manganese-antimony system are sensitive to the presence of interstitial cations. For example, Mn1+xSb phases have been reported ( ) with Curie temperatures varying between 300 and 90°C. Although MnSb is free from first-order phase changes, bulk samples of Mn1.2Sb were found to be magnetically unstable on thermal cycling through their Curie temperature. Repeated thermal cycling resulted in a shift of the Curie temperature from 90 to ∼300°C ; a value typical of stoichiometric MnSb. Lattice constant and magnetization measurements are consistent with this magnetic aging being associated with the migration of manganese ions away from the bipyramidal interstices in which they are known to couple antiferromagnetically with the octahedral lattice site manganese ions. Polycrystalline films of stoichiometric MnSb were prepared by RF sputtering and were shown to have optical absorption and Faraday rotation comparable to isostructural MnBi. Attempts to prepare films of Mn1.2Sb with a Curie temperature of 90°C always resulted in material with the magnetic properties of stoichiometric MnSb, and lattice constant data strongly suggest that we were unable to populate the interstitial cation sites in the films.