Oxidation　is　one　of　the　main　failure　modes　of　superalloys,　and　the　initial　oxidation　of　a　superalloy　provides　clues　for　its　failure　analysis.　NiCo-based　superalloys　exhibit　superior　mechanical　properties,　but　their　oxidation　behaviour　is　still　debatable,　especially　in　terms　of　the　ratio　of　Ni/Co.　Here,　we　show　the　effect　of　the　Ni/Co　ratio　on　the　initial　oxidation　behaviour　at　the　nanoscale　by　in　situ　environmental　transmission　electron　microscopy　(ETEM),　which　provides　the　initial　microstructural　changes　in　the　γ/γ′　structure-based　alloy.　The　results　indicate　that　Ni-rich　alloys　exhibit　better　structural　stability　at　high　temperatures　than　Ni-deficient　alloys.　A　high　content　of　Ni　promotes　the　formation　of　a　dense　oxide　layer,　inhibits　the　oxidation　rate,　and　improves　the　oxidation　resistance.　This　result　will　help　to　elucidate　the　basic　process　of　oxide　formation　in　NiCo-based　single-crystal　superalloys　and　provide　ideas　for　improving　their　oxidation　resistance　for　alloy　design.　©　2024　The　Author(s)