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The competition between Na+ and K+ with a protein for water was investigated by using Density Functional Theory (DFT) calculations. The optimized potential energy curves have been made in the DFT, together with balanced basis sets of split valence Def2-SV(P). Initially, calculations were done in order to know the organization of the hydration shell of the sodium and potassium ions, when up to sixteen molecules of water are added. The results indicate the structure and stability of these cations with water clusters. Then, this knowledge was used for the analysis of the hydrated protein when potassium or sodium cations approach them, showing that cation has a dehydration process more favorable energetically, and indicating for which cation, potassium or sodium is the competition with the protein for water more favorable. This work was partially supported by Spanish Ministry of Science and Technology Grant FIS2012-35880.