Polyborosiloxanes as a precursor for borosilicate gel plates and fibers were obtained by the reaction of silicic acid and boron tri-n-butoxide followed by alkoxylation with 1-butanol. Gel fibers were prepared after concentration and aging of the polymer solutions with molar ratios of silicic acid to boron tri-n-butoxide of 7/3-2/3. Further aging of the solutions gave gel plates with cracking, depending on boron content. Spectral and chemical analyses showed that the borosiloxane bond formation initiated during polymerisation reaction and is enhanced by heat treatment. (C) 1999 Elsevier Science B.V. All rights reserved.