Abstract.Cerium is exploited as a probe cation for elucidating the structure of an alkoxide-derived silica gel and its progressive evolution to a glass network as a function of heat-treatment up to 1000~ At intermediate temperatures, the host structure exhibits inhomogeneity due to insufficient formation of siloxane bonds, which is reflected by at least two different sites and co-ordination spheres (termed "high" and "low" water ligation) for cerium. This is proved by the response of the gels heated up to 700~ to rehydration. Further formation of Si-O-Si network (900~ leads to the destruction of the "high water" sites of cerium and progression towards a glassy structure. It is, however, only after heat-treatment at 1000~ that a dense silica glass network, not responding to rehydration, is finally obtained with cerium ions embedded in it.