We report an in situ non-destructive optical microspectroscopy method for measurement of the diffusion activation energy for silver in glass. The approach is based on a set of isothermal time-dependent characteristics of the surface plasmon resonance of Ag nanoparticles analyzed in the context of Mie extinction and crystal growth theories. The material studied in real time during thermal processing consisted of a melt-quenched phosphate-based glass system to which 4 mol% of Ag2O along with reducing agent SnO in the same amount were added. A diffusion activation energy of 5.2 eV was obtained and discussed in terms of glass composition. The mechanism of Ag nanoparticle growth was elucidated as well. (C) 2011 Elsevier B.V. All rights reserved.