Trimethylsilylation (TMS) reactions have been carried out on acid degradable fluoro-alumino-silicate glasses containing a basic oxide. The siloxanes produced by the TMS reactions were analysed in order to gain information about the silicate structure of the glass. The results indicate a non-random arrangement of SiO4 and AlO4 tetrahedra in the glasses studied. No fluorinated derivatives of siloxanes were found, indicating that fluorine atoms are not bonded to the silicon atoms of the glass network. It is thought that fluorine atoms are instead bonded to the aluminium atoms present. The bonding of fluorine to aluminium and not to silicon atoms explains the prevention of fluorine loss as silicon tetrafluoride (SiF4) from melts containing both aluminium and a basic oxide, and in addition explains the reduction in the glass transition temperature behaviour found on incorporating fluorine into alumino-silicate glasses. The results suggest that the environmental problem of silicon tetrafluoride loss from fluoro-silicate glass melts and its subsequent hydrolysis to hydrofluoric acid and silica can be avoided by including a basic oxide in the composition.