Reactivity of MgO surfaces towards dissociative adsorption of water is crucial in determining the protective performance of oxide films formed on top of Mg alloy surfaces. Engineers and scientists have repeatedly reported that heavy metal contamination could significantly affect the oxide film stability towards water. We performed first-principles analysis on MgO surfaces adsorbed with water molecules in the presence of six different types of cationic impurities – Zn 2+ , Al 3+ , Y 3+ , Mn 4+ , Zr 4+ , and Ce 4+ – to assess the effect of commonly present metal impurities in MgO films on the dissociative adsorption of water. The trend in the adsorption energies with different metal impurities shows that water is less likely to dissociate when cationic impurities with high oxidation states such as Mn 4+ , Zr 4+ , and Ce 4+ are present. Charge transfer from the adsorbed species to the impurity atom is identified as the source of the trend. Increased dissociation barrier for water adsorbed on the same doped surfaces indicates that the incorporation of such impurities will effectively block the dissolution of oxide films and potentially enhance the film's resistivity to water.