The mechanism of the high shrinkage that occurs in chromic oxide in reducing conditions has been investigated by micrographic, analytical, electrical resistance, and other means. The effect of the reducing conditions is to modify the surface of the oxide particles, the core remaining normal oxide (i.e. oxygen excess). The modification is predominantly a change to a lower oxide or oxides and/or metal, although transitional formation of oxygen-deficient oxide is implied. In oxidising conditions (which cause fritting, but not shrinkage), modification to the extent that surfaces become of approximately stoichiometric proportions also occurs, but this change is apparently caused by loss of trioxide and results in unreactive oxide.