A process for producing an aqueous silica-based sol is disclosed wherein a cationic ion exchange resin having part of its ion exchange capacity in hydrogen form is contacted with an aqueous alkali metal silicate to form a slurry having a pH from 5.0 to 11.5 and/or having particle aggregation or microgel formation corresponding to a S value up to 45%; adjusting the pH using a material comprising an aluminium compound; and separating the resin from the slurry. Silica-based sols having an S value from 15 to 25%, mole ratio Si:Al from 20:1 to 50:1, mole ratio Si:X, where X=alkali metal, from 5:1 to 17:1, SiO.sub.2 content of at least 5% by weight and containing silica-based particles having a specific surface area of at least 300 m.sup.2/g, as well as the use of such silica-based sols in producing paper are disclosed.