AbstractA technique was developed to aid in the interpretation of nucleation and growth characteristics of otherwise electron‐opaque Al2O3particles. The technique involved embedding a dense collection of particles in an Epon mixture and sectioning with a diamond knife on an ultramicrotome. Serial sections were utilized to obtain three‐dimensional information. It was found that ultramicrotomy does not introduce artifacts in ceramics which would otherwise affect measurements of the crystallite size or phase fraction, nor does it affect the crystallographic data significantly.It was observed that α‐Al2O3nucleated at the surface of isolated particles and in the neck region of sintered particles. Other data, obtained from diffraction and in situ hot‐stage studies, indicated that each polycrystalline α‐Al2O3particle transformed to a nearly single crystal of γ‐Al2O3, which extended into adjacent particles under favorable