Heterogeneous Cox Ag1−x and Nix Ag1−x alloys have giantmagnetoresistance properties. Such alloys, with atomic concentrations x = 0.20 and 0.37,were studied by transmission X-ray Absorption Spectroscopy at the Co or Ni K-edge afterin situ thermal annealing. For alloys as-deposited and annealed at 200 °C,Extended X-ray Absorption Fine Structure analysis displays both Co-Co (Ni-Ni) bonds relatedto Co (Ni) atoms agglomerated in magnetic particles and Co-Ag (Ni-Ag) bonds related to Co(Ni) atoms in substitutional sites in the Ag matrix. At the same alloy concentration, themiscibility in the Ag matrix is found larger for Ni than for Co. After annealing around250 °C, the marked decrease of the Ag neighbour peak corresponds to a diffusion ofmagnetic atoms outside the Ag matrix. The Co-Co coordination number increases regularly withannealing temperatures up to 450 °C reflecting a progressive expansion of Coparticles. On the contrary, for Ni alloys, no further particle expansion has been observedin the same annealing range. This different behaviour may be linked both to the differencebetween the as-deposited structural states and to the Co/Ag and Ni/Ag interfaces energies.Using a simple model, evolution of the mean particle size has been estimated as a functionof annealing.