This study investigates the oxidation mechanisms in a five-element equiatomic CoCrFeNiMn high entropy alloy (HEA) under controlled oxygen conditions. The research aims to understand how each element interacts with the environment and how these interactions influence the formation and evolution of oxide layers. The study employs in-situ atom probe tomography, transmission electron microscopy, and X-ray absorption near-edge structure techniques to analyze the oxidation process and surface oxide structure. The findings reveal that oxidation-induced surface changes are governed by thermodynamics and kinetics, with redox potential becoming dominant over time, leading to composition inversion. The study also highlights the role of atomic size, diffusivity, and redox potential in determining the oxidation behavior of each element. The results show that Cr and Mn oxidize first due to their lower oxidation potentials, followed by Ni, Fe, and Co. The oxidation process leads to the formation of a layered oxide film with Cr-rich and Mn-rich oxides. The study also demonstrates that the addition of Al to the HEA alters the oxidation behavior, leading to the formation of a more complex oxide structure. The research provides a mechanistic understanding of speciated oxide growth in HEAs, which can guide the design of complex alloys with improved oxidation resistance under extreme conditions. The study underscores the importance of understanding the oxidation behavior of HEAs for their application in high-temperature environments.This study investigates the oxidation mechanisms in a five-element equiatomic CoCrFeNiMn high entropy alloy (HEA) under controlled oxygen conditions. The research aims to understand how each element interacts with the environment and how these interactions influence the formation and evolution of oxide layers. The study employs in-situ atom probe tomography, transmission electron microscopy, and X-ray absorption near-edge structure techniques to analyze the oxidation process and surface oxide structure. The findings reveal that oxidation-induced surface changes are governed by thermodynamics and kinetics, with redox potential becoming dominant over time, leading to composition inversion. The study also highlights the role of atomic size, diffusivity, and redox potential in determining the oxidation behavior of each element. The results show that Cr and Mn oxidize first due to their lower oxidation potentials, followed by Ni, Fe, and Co. The oxidation process leads to the formation of a layered oxide film with Cr-rich and Mn-rich oxides. The study also demonstrates that the addition of Al to the HEA alters the oxidation behavior, leading to the formation of a more complex oxide structure. The research provides a mechanistic understanding of speciated oxide growth in HEAs, which can guide the design of complex alloys with improved oxidation resistance under extreme conditions. The study underscores the importance of understanding the oxidation behavior of HEAs for their application in high-temperature environments.