The performance of metal oxide-based battery active materials is compromised by surface contaminants formed during storage and handling in ambient air. We present a detailed analysis of the true nature and the quantity of these surface contaminants on three different cathode active materials (CAMs), the widely used LiNi_1/3Mn_1/3Co_1/3O₂ (NMC111), the high surface area Li- and Mn-rich xLi₂MnO₃•(1-x)LiNi_aMn_bCo_cO₂ (HE-NMC), and the Ni-rich LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) and LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811). We process these materials in three distinct conditions “dry” (as-received materials dried at 120 °C), “wet” (excessive air exposure at high relative humidity) and “calcined” (high temperature treatment) and identify surface contaminants by X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis coupled with mass spectrometry (TGA-MS) and diffusive reflectance infrared fourier transform spectroscopy (DRIFTS). We test the CAMs’ reactivity with an ethylene carbonate-based electrolyte using on-line electrochemical mass spectrometry (OEMS) and evaluate their performance in Li-ion battery cells. We demonstrate that not only the commonly assumed LiOH and Li₂CO₃ residues account for performance deterioration, but also transition metal hydroxides/carbonates formed on the material surface during exposure to atmospheric moisture and CO₂. Finally, we showcase a thermal treatment that removes these transition metal based surface contaminants and leads to superior cycling stability.