[en] Electron diffraction and microscopy studies supplemented by electron spectroscopic techniques such as Auger electron spectroscopy and energy dispersive x-ray spectroscopy were used to characterize the nature of grain boundary segregation in commercial grade (Mn,Zn)Fe₂O₄ samples containing small quantities of CaO. Chemical analyses by AES and EDAX show an enrichment of Ca near the grain boundary region. Convergent beam electron diffraction experiments show that the crystal symmetry of the spinel structure is distorted in the vicinity of the grain boundary. In-situ heating experiments in HVEM show the existence of a disordered phase at the sintering temperature. Lorentz microscopy in TEM shows the interaction of magnetic domain wall motion with grain boundaries. These chemical and structural features are correlated with electrical resistivity and magnetic permeability of the ferrites. 4 figures