In this work, high resolution integrated AFM–EC/SECM was used to reveal the spatially heterogeneous electroactivity of microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) surfaces. During electrochemical corrosion, NCD surfaces undergo a stronger corrosion reaction than MCD because of the higher amount of sp2 hybridized carbon. In-situ EC-AFM imaging shows no significant change in surface morphology, while corroded surfaces become more hydrophilic due to the oxidation reactions that occur in the outermost layer. On non-corroded MCD and NCD surfaces, intercrystallite boundaries exhibit stronger localized (electro)chemical reactivity than crystallites. However, after electrochemical corrosion, both MCD and NCD surfaces become thermodynamically stabilized by corrosion products that passivate the surface and inhibit further corrosion. In this way, the (electro)chemical reactivity of the intercrystallite boundaries is reduced to a greater extent by electrochemical corrosion than the (electro)chemical reactivity of the crystallites due to the more intense electrochemical oxidation reactions taking place at these boundaries. After corrosion, this results in a comparatively greater (electro)chemical reactivity on the crystallites than at the boundaries. This behavior suggests the following order of (electro)chemical reactivity: sp2 > sp3 > oxidized/passivated structures.