Magnetic field effects on silica gel formation from aqueous silicate solutions were examined by Si-29 NMR, viscosity, and N-2 adsorption measurements. Si-29 NMR spectra were analyzed by the multivariate curve resolution to give three principal spectra, which may correspond to oligomeric species, polymeric species, and microgels. A 6 T magnetic field promoted the formation of polymeric species and the transformation of polymeric species to microgels. In addition, the magnetic field reduced the viscosity of the silicate solutions, suggesting depression of particle growth and interactions among silanol groups on particles under magnetic fields. Such magnetic field effects may relate to magnetic orientation and hydration of polymeric species and microgels.