Samples of BaCO3 + 4Fe(3)O(4) mixtures were microwave irradiated in an 83 GHz gyrotron beam system and a 2.45 GHz single mode system. Twenty seconds of irradiation with 2-6 kW and 83 GHz energy lead to the formation of the BaFe12O19 form in major quantities, with unreacted Fe2O3 in minor quantities. A combination of 3 kW power and 90 s irradiation time resulted in selective absence of some X-ray diffraction (XRD) peaks and considerable reduction of other XRD peak intensities, suggesting that the BaFe12O19 surface is partially decrystallized. However, in a 2.45 GHz H-field system, irradiation of the BaCO3 + Fe3O4 mixture resulted in a total disappearance of peaks in the XRD, establishing complete decrystallization in 5 s. The 83 GHz effects appear to be mainly on the surface of the materials processed, whereas for the mixture reacted in the 2.45 GHz H-field the decrystallization effect occurs in bulk. These findings correlate well with the greatly different absorption lengths at the two frequencies. (C) 2004 Elsevier B.V. All rights reserved.