Platinum dissolution and restructuring due to surface oxidation are primary degradation mechanisms that limit the lifetime of platinum-based electrocatalysts for electrochemical energy conversion. Here, we have studied well-defined Pt(100) and Pt(111) electrode surfaces by in situ high-energy surface X-ray diffraction, online inductively coupled plasma mass spectrometry and density functional theory calculations to elucidate the atomic-scale mechanisms of these processes. The locations of the extracted platinum atoms after Pt(100) oxidation reveal distinct differences from the Pt(111) case, which explains the different surface stability. The evolution of a specific oxide stripe structure on Pt(100) produces unstable surface atoms that are prone to dissolution and restructuring, leading to one order of magnitude higher dissolution rates.