Samples of 40SiO 2·30Na 2O·1Al 2O 3·(29 - x)B 2O 3· xFe 2O 3 (mol%), with 0.0 ≤ x ≤ 17.5, were prepared by the fusion method and investigated by electron paramagnetic resonance (EPR), optical absorption (OA) and Mössbauer spectroscopy (MS). The EPR spectra of the as-synthesized samples exhibit two well-defined EPR signals around g = 4.27 and g = 2.01 and a visible EPR shoulder around g = 6.4, assigned to isolated Fe 3+ ion complexes (g = 4.27 and g = 6.4) and Fe 3+-based clusters (g = 2.01). Analyses of both EPR line intensity and line width support the model picture of Fe 3+-based clusters built in from two sources of isolated ions, namely Fe 2+ and Fe 3+; the ferrous ion being used to build in iron-based clusters at lower x-content (below about x = 2.5%) whereas the ferric ion is used to build in iron-based clusters at higher x-content (above about x = 2.5%). The presence of Fe 2+ ions incorporated within the glass template is supported by OA data with a strong band around 1100 nm due to the spin-allowed 5E g- 5T 2g transition in an octahedral coordination with oxygen. Additionally, Mössbauer data (isomer shift and quadrupole splitting) confirm incorporation of both Fe 2+ and Fe 3+ ions within the template, more likely in tetrahedral-like environments. We hypothesize that ferrous ions are incorporated within the glass template as FeO 4 complex resulting from replacing silicon in non-bridging oxygen (SiO 3O -) sites whereas ferric ions are incorporated as FeO 4 complex resulting from replacing silicon in bridging-like oxygen silicate groups (SiO 4). © 2012 Elsevier Masson SAS. All rights reserved.