Monodisperse silica particles were prepared in microemulsion media using block copolymers and nonionic surfactants. The silica particles were obtained by hydrolysis and condensation of tetraethoxysilane, using Mis as a catalyst, and they were characterized by optical and electron microscopy, nitrogen sorption, Fourier transform infrared spectroscopy and thin-layer chromatography. Particles, with average sizes from 0.1 to 1.5 mu m and with specific surfaces lower than 100 m(2)/g could be obtained. The results showed that the size could be controlled by both the cosurfactant chain length and concentration. The higher the cosurfactant concentration and the shorter the chain length, the larger the particles. This has been attributed to the increase of interfacial flexibility with reduction of chain length. This causes an increase in the velocity of particle growth, then producing larger particles.