Mullite fibre was successfully synthesized by pyrolysis of aluminosiloxane formed from ethyl 3-oxobutanoatodiisopropoxyaluminium and di-(sec-butoxy)aluminoxytriethoxysilane. Aluminosiloxane increased in viscosity with increasing coordination number of aluminium, which was analysed by Al-27 NMR spectroscopy. The viscosity of aluminosiloxane was controlled by the amount of added glacial acetic acid as well as the working temperature for the spinning of precursor fibre. Aluminosiloxane gave amorphoUS SiO2-Al2O3 at 500-degrees-C, which began to crystallize to mullite at 930-degrees-C. Single-phase mullite was produced on heating to 1000-degrees-C for 1 h. The polymer fibre spun from aluminosiloxane was pyrolysed yielding crack-free mullite fibre at 1000-degrees-C for 1 h.