For the cathode of a proton exchange membrane fuel cell (PEMFC), the current leading industrial electrocatalyst is pure platinum for the oxygen reduction reaction (ORR). The ORR reduces a stream of gaseous oxygen over an electrocatalyst to form water and complete the electrical circuit. Even though platinum has been found to have the highest activity for the ORR for pure catalysts, this activity is orders of magnitude lower than that found for other general electrode reactions such as the hydrogen oxidation reaction (HOR). It has been shown previously in literature that the activity can be increased if the 2nd layer of a platinum surface is replaced with a 3d transition metal giving a subsurface bimetallic catalyst. In the current study, we attempt to quantify the stability of the Pt-3d-Pt(111) subsurface electrocatalysts under an oxygen environment. Experiments were performed for the Pt-Ni and Pt-Co bimetallic systems using ultra-high vacuum (UHV) techniques. The segregation of Ni and Co was verified using high-resolution electron energy loss (HREELS). The activation barrier for the segregation of the 3d transition metal to the surface was determined using Auger electron spectroscopy (AES). The remaining Pt-3d bimetallic systems were compared using predicted thermodynamic stability calculated using density functional theory (DFT). Kinetically, the Pt-Ni subsurface configuration was determined to be more stable than the Pt-Co subsurface configuration when exposed to oxygen. Thermodynamically, the Pt-Ni subsurface configuration has been predicted to be the most stable of the Pt-3d subsurface configurations when exposed to oxygen.