The properties of sol-gel synthesis of borosilicate sols, wet gels and xerogels, as well as thin nanosized films were studied. Physicochemical processes and phenomena accompanying sol-gel synthesis of borosilicate sols, gels, xerogels, and thin spin-on glass films were analyzed on the basis of reference data and our long-term experience. Particularly, the sol-gel synthesis of borosilicate materials with high boron concentration were described in detail. The morphology, mesostructures and chemical composition of sol-gel derived borosilicate composites with a high boron content (from 30 to 48 wt %, based on B2O3) were studied using a number of complementary research methods (optical, scanning and transmission microscopy, small-angle X-ray scattering, and FTIR spectroscopy). The effect of sol composition (concentrations of Si(OEt)4 and H3BO3), of the addition of polyols (primarily glycerol), of the conditions for synthesis and of aging of target materials on their composition were studied, as well as the morphology and mesostructures of the composites