Kinetics of polymorphic transition of silver selenide is presented : Sliver selenides formed at both low temperature by beta-Ag2Se (coprecipitation samples) and at high temperature by alpha-Ag2Se (single crystals) were investigated to show a Ag-trapping phenomenon i.e. the formation of Ag clusters at Ag atomic traps. DSC (differential scanning calorimetry) signals and resistivity data explain the formation of Ag clusters in the cooling process and their dissolution in the heating sequences. The trapping phenomenon of the coprecipitation samples, which were prepared at the beta-phase, was affected by phase transition. The Ag trapping phenomenon at the beta-phase of a single crystal, quenched from a phase with excess Ag, was also observed. The cooling process relates to the kinetics of the phase transition itself and the cluster formation which is affected by lattice defects and the concentration of non-stoichiometric Ag atoms.