Solid state 29Si and 31P MAS NMR have been used to investigate the microstructural changes occurring in phosphosilicate gels during their conversion from a gel to the corresponding gel-derived glasses by heating. The studied gels have the molar compositions 10P2O5·90SiO2 and 30P2O5·70SiO2. It was found that the dried gels (100 °C) have very similar structures formed by a siloxane framework containing silanol groups and including trapped molecules of orthophosphoric acid along with a very small amount of pyrophosphoric acid. In spite of this initial similarity further heating causes markedly different structural rearrangements of their glassy matrices. Namely, the co-polymerisation of phosphate and silicate tetrahedra takes place at 300 °C for the gel with the higher phosphorus content whereas this occurs only after heat treatment for 30 minutes at 400 °C for the gel with the lower phosphorus content. Moreover, the presence of six-coordinated silicon in the glassy matrix of the 30P2O5·70SiO2 gel has been observed. The different evolution of the investigated gel microstructures mirrors their different crystallization behaviour: 10P2O5·90SiO2 keeps its amorphous nature up to 1000 °C while for 30P2O5·70SiO2 crystallization starts after heat treatment for 30 minutes at 400 °C.