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The microstructures of differently processed Commercial grade MnZn soft ferrites have been studied using transmission electron microscopy and microanalysis, and the microstructural features correlated with the magnetic properties. Special emphasis has been placed on the grain boundaries. The permeabilities range frOm 1200 to 18,000 and loss factors (tan5/p) from 1.0 x 106 to 13.7 x 106 at 50 kHz. The pore distribution in these ferrites was quite diverse although the total porosities are nearly the same. In specimens of low i either the defect density was high or varying amounts of amorphous phase were observed at the grain boundaries. In these samples STEM X-ray analyses revealed the presence of Ca and Si in the thin grain boundary phase Also, in some specimens the presence of Fe20 3 and/or Mn0 2 precipitates at some grain junctions was observed In the high p ferrites, few defects and no intergranular ohases were observed, and Ca 12 could not be detected It is suggested that the lattice distortion associated with the amorphous phase and the boundary seqregation reduces the permeability.