The reaction products of an allophane heated with carbon at 850-1600 degrees C in the stream of nitrogen for a given time were characterized by X-ray diffractometry. As a result, it was found that cristobalite and mullite were stable phases at 850-1300 degrees C, beta-Si3N4 and alpha-Al2O3 at 1300-1500 degrees C, and SiC-AIN-Fe2Si at temperatures higher than 1500 degrees C. SiC-AIN-Fe2Si composites with high porosity of about 50% were easily prepared by a heat treatment at a temperature higher than 1500 degrees C with carbon in a stream of nitrogen. The formation mechanism of the composites is kinetically discussed from a viewpoint of small-pore shrinkage and large-pore expansion by volume diffusion during heating. The resultant microstructure of the composites obtained is also discussed.