Electron magnetic resonance (EMR) spectra of gadolinium-doped strontium titanate (SrTiO3) have been studied at room temperature for gadolinium concentrations between 0.20 and 1.20 mol%. The results suggest that the Gd3+ ions occupy substitutional sites, replacing the Sr2+ ion, that the electron magnetic resonance linewidth increases with increasing gadolinium concentration and that the range of the exchange interaction between Gd3+ ions is about 0.96 nm, of the same order as that of the same ion in other host lattices, such as ceria (CeO2), quicklime (CaO), strontia (SrO) and zircon (ZrSiO4). The fact that the electron magnetic resonance linewidth of the Gd3+ ion increases, regularly and predictably, with Gd concentration, shows that the Gd3+ ion can be used as a probe to study, rapidly and non-destructively, the crystallinity and degradation of SrTiO3. (C) 2011 Elsevier B.V. All rights reserved.