The method consists in hydrolysing an initial volume Vse of a precursor material comprising at least one polyalkoxysilane with a quantity of water such that.times..times..times..times..times..gtoreq..times. ##EQU00001## where x H.sub.20 and x Si represent the number of moles of H.sub.20 and Si present, respectively, the concentration of the hydrolysate up to a volume substantially equal to the initial volume Vsi, leaving the concentrated hydrolysate until segregation into an aqueous phase and an organo-silicon phase, and recovery of the organo-silicon phase. A product can be obtained which comprises an organo-silicon sol comprising silicon species T.sup.1, T.sup.2, and T.sup.3, wherein the molar fraction of species T.sup.2 is equal or greater than 50%, comprising a condensation rate equal to or greater than 0.65, and further defined as lacking water, as determined by analyzing a sample of said organo-silicon sol by .sup.1H NMR and observing the absence of peak corresponding to water.