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Carbon 1s ionization energies have been measured for 12 halomethanes. These together with earlier measurements provide 27 compounds for investigating the relationship between core-ionization energies and the electronegativity and hardness of the halogens. The ionization energies correlate nearly linearly with the sum of the electronegativities of the halogens attached to the central carbon. Both electronegativity and hardness play important roles in determining the ionization energy, and it is found that the linear relationship between ionization energy and electronegativity arises from an interplay of the electronegativity and hardness of the halogens and the length and ionicity of the carbon-halogen bond.