The microstructure of some Fe-Si alloys water-quenched from 700-1200degreesC and aged at 550-675degreesC for various times has been studied by means of transmission electron microscopy (TEM). It has been shown that at temperatures of 700degreesC and above the microstructure of the alloy is a solid solution with two-dimensional particles of a B2 ordered phase. These particles have a FeSi composition and lie along (100) planes of the matrix. At temperatures of 675degreesC and below a modulated microstructure is formed; in its enriched modulations the composition of the Fe3Si phase is attained and D0(3) ordering occurs. Phase composition of some high-silicon alloys has been determined by X-ray diffraction phase analysis. It corresponds to alpha + B2 or alpha + D0(3). These results are plotted on the accepted Fe-Si phase diagram. (C) 2004 Kluwer Academic Publishers.