The Stöber process is a sol-gel approach to preparing monodisperse (uniform) spherical silica () materials that was developed by a team led by Werner Stöber and reported in 1968. The process, an evolution and extension of research described in Gerhard Kolbe's 1956 Ph.D. dissertation, was an innovative discovery that still has wide applications more than 50 years later. Silica precursor tetraethyl orthosilicate (, TEOS) is hydrolyzed in alcohol (typically methanol or ethanol) in the presence of ammonia as a catalyst:
The reaction produces ethanol and a mixture of ethoxysilanols (such as , , and even ), which can then condense with either TEOS or another silanol with loss of alcohol or water:
Further hydrolysis of the ethoxy groups and subsequent condensation leads to crosslinking. It is a one-step process as the hydrolysis and condensation reactions occur together in a single reaction vessel.
The process affords microscopic particles of colloidal silica with diameters ranging from 50 to 2000 nm; particle sizes are fairly uniform with the distribution determined by the choice of conditions such as reactant concentrations, catalysts, and temperature. Larger particles are formed when the concentrations of water and ammonia are raised, but with a consequent broadening of the particle-size distribution. The initial concentration of TEOS is inversely proportional to the size of the resulting particles; thus, higher concentrations on average lead to smaller particles due to the greater number of nucleation sites, but with a greater spread of sizes. Particles with irregular shapes can result when the initial precursor concentration is too high. The process is temperature-dependent, with cooling (and hence slower reaction rates) leading to a monotonic increase in average particle size, but control  distribution cannot be maintained at overly low temperatures.