By combining a plasmonic metal with a reactive one, nanoalloys become valuable systems for optical purposes by using the Localized Surface Plasmon Resonance (LSPR) as the probe for chemical processes. It is thus crucial to develop experimental devices allowing the in situ or in operando monitoring of these nanoparticles in order to identify the mechanism involved during restructuring processes. For this purpose, we developed a new setup allowing in situ optical measurements of a nanoparticle (NP) assembly, complementary to single NP environmental transmission electron microscopy (ETEM) observations [1]. The setup was tested on iron-silver NPs, which combine a plasmonic material with a ferromagnetic element [2]. A detailed study of the Ag-Fe bimetallic system was thus conducted for two silver compositions (50% and 80%), by investigating the chemical properties of the NPs through TEM and environmental optical spectroscopy, and interpreting the combined physicochemical observations with the help of parallel tempering Monte Carlo (MC) simulations of their structural stability at finite temperature. We show that these metals are initially segregated with a silver-enriched surface and that oxidation induces some iron migration towards the surface. In situ optical measurements carried out successively under oxidizing and reducing atmospheres indicate that iron oxidation and reduction are reversible processes that are directly mirrored in the LSPR changes (see Figure 1), in good agreement with optical simulations. Based on these results we propose a scenario of the evolution of the chemical ordering under reducing and oxidizing atmospheres. REFERENCES 1 Julien Ramade, Cyril Langlois, Michel Pellarin, Laurent Piccolo, Marie-Ange Lebeault, Thierry Epicier, Mimoun Aouine, and Emmanuel Cottancin, Tracking the restructuring of oxidized silver-indium nanoparticles under a reducing atmosphere by environmental HRTEM, Nanoscale, 2017, 9, 13563-13574 2 Julien Ramade, Emmanuel Cottancin, Marie-Ange Lebeault, Cyril Langlois, Laurent Piccolo, Michel Broyer, Matthias Hillenkamp, Jean Lermé, Florent Calvo and Michel Pellarin, Environmental Plasmonic Spectroscopy of Silver-Iron Nanoparticles: Chemical Ordering Under Oxidizing and Reducing Conditions. Journal of Physical Chemistry C 2019, in press.