Abstract--X-ray powder diffraction (XRD), transmission electron microscopy, infrared spectroscopy, differential thermal analysis, and surface area (BET) measurements were employed to investigate the transformation of microcrystalline maghemite to hematite. At 500~ pure maghemite was completely altered to hematite in 3 hr, whereas maghemites containing small amounts (< 1%) of Co, Ni, Zn, Cu, Mn, A1, V, and Cr required much longer heating times. The maghemite-to-hematite transformation temperature varied from 540* to 650"C. XRD line widths suggest that each particle of maghemite and hematite may have been a mosaic of many independent, coherently diffracting crystals. The transformation of maghemite to hematite at 650~ was accompanied by a reduction in surface area due to sintering of particles.