Polyaluminocarbosilane (PACS) as the precursor of high-temperature resistant SiC fibers was synthesized by reacting polycarbosilane (PCS) with aluminum(III)acetylacetonate [Al(AcAc)(3)] at 310degreesC in N-2 under ambient pressure. The reaction mechanism and the structure of PACS were investigated in detail by FTIR, GPC, GC/MS, ESCA, and elemental analysis. The reaction was proven complex involving the formation of Si-O-Al and Si-Al-Si bonds, which were accompanied by the evolution of 3-methoxy-2,2-dimethyl-oxirane, 2,3-dihydro-[1,4]dioxine, pent-3-en-2-one, and 3-ethyl-but-3-en-2-one, and acetylacetone.