The amorphous aluminosilicate allophane was synthesized by rapid mixing of inorganic solutions with high initial concentrations (10–100 mmol/l) followed by hydrothermal treatment. X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed homogeneous products having a hollow spherical amorphous structure with a particle diameter of 3–5 nm. The amorphous products had a high BET specific surface area (490–552 m2/g) in comparison with natural allophane and had a narrow pore-size distribution (2–5 nm in diameter). The results of water vapour adsorption isotherm studies showed a gradual increase over the range of relative water vapour pressure of 0.6–0.9 and reached a maximum of ~85 wt.%. The synthetic allophane shows promise as an adsorbent material because of its high adsorption-desorption capacity and its unique structure.