The HF etching of a silicon(boron)oxycarbide glass was, for the first time, followed in situ by synchrotron radiation X-ray diffraction. The starting Si(B)OC glass is formed by two interpenetrating nano-sized networks, one SiO2-rich and the other C-rich comprising β-SiC nanocrystals and nanocrystalline graphite. The silica-based phase is dissolved by the HF forming a porous C-rich Si(B)OC. The XRD analysis shows the quantitative evolution with the etching time of all nanocrystalline phases, including the amorphous SiO2-based one. The rate-limiting step for the dissolution is the diffusion of the reactants/products through the porous C-rich Si(B)OC. The XRD analysis shows also a trend of the basal spacing of the sp2 carbon phase as a function of the dissolution time suggesting the presence, in the un-etched glass, of a in-plane residual compressive stress which is relaxed by etching out the SiO2–based network. At the end of the dissolution process the BET analysis shows the formation of a micro-meso porous C-rich Si(B)OC with a SSA of 595 m2/g, mesopore size in the range 2-10 nm and pore volume of 0.863 cm3/g.