Nanoparticles of Pt/Ni alloys represent state of the art electrocatalysts for fuel cell reactions. Density functional theory (DFT) based calculations along with in situ X-ray absorption spectroscopy (XAS) data show that the surface structure of Pt3Ni nanoparticulate alloys is potential-dependent during electrocatalytic reactions. Pt3Ni based electrocatalysts demonstrate preferential confinement of Ni to the subsurface when the electrode is polarized in the double layer region where the surface is free of specifically adsorbed species. Hydrogen adsorption triggers nickel segregation to the surface. This process is facilitated by a high local surface coverage of adsorbed hydrogen in the vicinity of the surface confined Ni due to an uneven distribution of the adsorbate(s) on the catalyst's surface. The adsorption triggered surface segregation shows a non-monotonous dependence on the electrode potential and can be identified as a breathing of the catalyst as was proposed previously. The observed breathing behavior is relatively fast and proceeds on a time scale of 100-1000 s.