The ancient mirror amalgam is a two-phase system: crystals of tin-mercury compounds surrounded by a mercury-rich liquid phase. Corrosion of the amalgam mirrors produces tin dioxide and tin monoxide and releases liquid mercury from the solid phase. The objectives of this study were to characterise the formation of the SnO2 nanometric particles in the alteration processes of ancient amalgam mirrors. Using grazing incidence X-ray diffraction, a depth profile analysis of the sample was performed. The morphology of the amalgam layer was studied by scanning electron microscopy (SEM), and transmission electron microscopy (TEM) was used to study the size and morphology of the particles. Elemental analysis of the amalgam was done by energy dispersive X-ray spectrometry (EDX). The SnO2 phase was straightforwardly identified by XRD using different incidence angles. The average crystalline size of the nanoparticles was evaluated using the Scherrer formula and was estimated in the range of 4 to 5 nm, which was in good agreement with the size estimated by TEM. The electron diffraction pattern of the nanoparticles could be indexed to the cassiterite (SnO2) structure, which is the most typical and stable corrosion product of tin.