The microstructure of phosphosilicate gel prepared by a sol gel process was investigated as a function of the heat-treatment temperature. Crystalline phases, Si3(PO4)4 and SiP2O7, were identified in the heat-treated phosphosilicate gel containing 56 mol % P2O5. It was found from the Fourier transform infrared spectra that phosphorus enters into the copolymer structure at approximately 600-degrees-C. The specific surface area of the gel increased with the heat-treatment temperature. An increase of the density of open pores with the heat-treatment temperature was observed in the secondary electron micrographs.