Metastable pyrophoric wustite (fexo, with x between 0.89 and 0.95) Was found to be much easier to grind to colloidal state than were its stable disproportion products, magnetite or iron. The grinding of polycrystalline magnetite appears to be complicated by the magnetic flocculation arising from interparticle attraction. This is superimposed on the attraction of a fluctuating electric dipole for a neighboring induced dipole. More than 85 percent of the tumbling time was saved by grinding antiferromagnetic wustite instead of the ferrimagnetic oxide of iron (fe3o4). Disproportionation of colloidal wustite to the stable ferromagnetic and ferrimagnetic colloidal particles of alpha-iron and magnetite, respectively, was accomplished rapidly at temperatures above 250 deg c but below 570 deg c, the eutectoidal stability point of the wustite phase.