Vanadyl isopropoxide can be polymerized in microemulsions of Aerosol OT containing water within reverse micelles. After a fast hydrolysis, vanadium pentaoxide forms ribbons in an intramicellar process taking about 100 s. These ribbons then aggregate by an intermicellar agglomeration taking 10(4) s. Drying these clusters in the presence of the surfactant leads to solids with surface areas around 100 m(2)/g.