- Abstract Two types of silica glass doped by o-phenanthroline (o-phen) (S2) and without organic reagent (S1) were studied. Paramagnetic probes (Mn(II), VO(II), Cu(II)) were introduced into both systems by doping or by impregnetion. Sol-gel-xerogel transition in these systems was investigated by electron paramagnetic resonance (EPR). In the S1 system, hydrated metal ions were shown to react weakly with silanol groups during every stage of silica polymerization. The effect of sol to gel7 transition is not observed in S1 spectra. Contrary to that, EPR spectra of VO(II) ions in S2 demonstrate significant changes during sol-gel-xerogel transition. As polymerization proceeds, the signals of non-immobilized hydrated metal ion and that of mixed water-o-phen-metal complex are substituted by signals of metal-o-phen complexes immmobilized on the porous surface. After gelation, the latter signals prevail. EPR spectra provide a means to monitor in situ structural changes during the various stages of glass formation.