An amorphous nanoparticulate aluminosilicate 3/2-mullite precursor has been synthesized and carefully characterized. The sol contained 2-nm particles of Q3(3Al) silica species together with six-coordinated alumina, which suggested an allophane-like structure of the nanoparticles. The sol remained stable for years, and formed an easily redispersible physical gel upon solvent evaporation. The gel crystallized to mullite at temperatures below 1000 °C, without going through any intermediate spinel phase. Thus, the nanoparticulate precursor is regarded as a homogeneous high-purity mullite precursor with a high Si–O–Al bond density, which is useful in the preparation of various nanostructured Al-rich aluminosilicate materials. The sols and gels were characterized by small-angle x-ray scattering, dynamic light scattering, x-ray diffraction, 27Al and 29Si magic-angle spinning (MAS) nuclear magnetic resonance spectroscopy, and differential thermal analysis.