We investigated the structure and surface chemistry of bimetallic PtAg nanoparticles, which were prepared by a water-in-oil microemulsion synthesis procedure, as well as the changes therein induced by electrochemical (surface) de-alloying, by a combination of electrochemical and in situ FTIR and online differential electrochemical mass spectrometry (DEMS) measurements. Based on transmission electron microscopy the resulting nanoparticles have a narrow size distribution with a mean diameter of around 6 nm, and increasing Ag contents lead to a broadening of the particle size distribution and a loss of the preferential formation of {100} facets observed for Pt nanoparticles. Evaluation of the hydrogen adsorption and CO oxidation characteristics and of the CO adsorption properties (IR) reveals detailed information on the surface composition of the bimetallic nanoparticles before and after electrochemical de-alloying, including also the distribution of Pt and Ag atoms in the surface. Furthermore, relevant information about the modifications in the chemical properties of Pt atoms/sites induced by the presence of Ag is extracted. For the as-prepared nanoparticles we find Ag surface enrichment, while de-alloying results in a core-shell structure with a PtAg core and a Pt shell, whose chemical properties are close to, but not identical, with those of polycrystalline Pt. More general, the study demonstrates that H-upd measurements are not suitable to identify the Pt surface content in such kind of bimetallic nanoparticles. (C) 2016 Elsevier B.V. All rights reserved.