[en] Mixtures of TEOS with substituted methoxysilanes generate low-density xerogels due to a nucleation mechanism involving the substituted alkoxysilane. The sol-gel transition of these mixtures was followed by theological characterisation. The transition from sol to gel takes place in a few minutes at ambient temperature. For the series exhibiting nucleation by the additive. the gel time goes through a slight minimum when the ratio of additive/main reagent increases. The elastic modulus increases with increasing ratio of additive/main reagent as the particle size decreases because of the nucleation mechanism by the additive. Samples with smaller particles exhibit the highest modulus for equal silica concentrations. (C) 2003 Elsevier Science B.V. All rights reserved.