Natural rubber composites containing silica particles generated in situ by the sol-gel process are investigated. The sol-gel processing is conducted before and after the cross-linking reaction. A good dispersion of the inorganic particles is obtained when the sol-gel reaction takes place after curing. In each case, the level of reinforcement is assessed from the mechanical and orientational behaviors. The reinforcement provided by in situ silica is compared with that obtained with anisotropic particles such as nanofibers of sepiolite. © 2005 Elsevier Ltd. All rights reserved.