The formation of the Guinier-Preston zones in aluminium alloys is closely linked with the excess vacancies. Traces of tin added to an Al-Ag alloy exert an influence on the Guinier-Preston zones precipitation. Due to their high binding energy with vacancies, tin atoms trap some of these available to promote the diffusion of silver atoms for the formation of the Guinier-Preston zones. At 125°C, tin microalloying slows down the reaction of the Guinier-Preston zones precipitation. The diffusion coefficient of the solute atoms in the Sn free alloy and in the Sn added alloy are determined during the coarsening regime which obeys to the Lifshitz, Slyosov and Wagner theory.Keywords: Guinier-Preston zones; precipitation; coarsening; diffusion; vacancies; hardening 1 Introduction Al-Ag supersaturated solid solution evolves towards the equilibrium state following the sequence [1-4]: Supersaturated solid solution → Guinier-Preston (GP) zones → metastable γ' phase → equilibrium γ phase. The Guinier-Preston zones (GP), consisting of silver atom clusters, are coherent with the matrix. The metastable phase γ'(Ag2Al) is semi-coherent with the matrix and the equilibrium phase γ(Ag2Al), is incoherent with the matrix. Precipitation starts from the formation of GP zones, which are isomorphous with the matrix and, therefore, have a lower interfacial energy than intermediate or equilibrium precipitate phases that possess a distinct crystal structure. As a result, the nucleation barrier for GP zones is significantly smaller. The effect of microalloying elements on the behaviour of age-hardenable alloys such as Al-Ag is an interesting physical problem addressing the mechanisms of transport and aggregation of the solute. Trace elements have been found to exert a disproportionate influence on the structure and properties of Al alloys compared with the amounts added which may be less than 0,1 molar fraction . Most trace element effects arise because they modify the nucleation and the growth characteristics of the phases which form during precipitation [5][6][7][8][9]. It is well known that the formation of the GP zones in aluminium alloys is closely linked with the excess vacancies. A number of models of GP zones precipitation assisted by vacancies has been developed by several authors [10][11][12]. GP zones formation is governed by a transport mechanism of solute atoms by solute atom-vacancy complexes. The high binding energy between tin atoms and vacancies leads to the formation of vacancy-tin atom pairs and silver atom-vacancy-tin atom complexes.