Some of the materials used as chromatography packings, adsorbents or catalyst supports are glasses of controlled porosity (CPGs). The chemical structure of these materials can easily be changed by heating the porous glass. Such thermal modification leads to the enrichment of the CPG surface in boron atoms. The long exposure of CPGs to thermal treatment can even cause the formation of borate crystals in the pores of the glasses. This paper considers the application of thermal analysis to the investigation of porous glasses heated for different periods of time.