Free energy perturbation (FEP) calculations using the Amber 95 force field and the TIP3P water model were carried out to evaluate the solvation free energy of deoxyribonucleoside triphosphates in aqueous solution. Solvation free energies of -307.5, -311.5, -314.1, and -317.0 kcal/mol were calculated for the (Mg, dTTP)(2-), (Mg, dATP) 2-, (Mg, dCTP) 2-, and (Mg, dGTP) 2- complexes, respectively. Structural origins of the relative solvation free energies of deoxyribonucleoside phosphates were examined by calculating the contribution of the interaction of the base moiety with its surroundings. We showed that for each nucleobase the magnitude of this contribution is unaffected by substituting the 5'-OH group of the corresponding nucleoside with the charged mono- or triphosphate groups. This free energy contribution was further decomposed into the sum of free energies originating from the interactions of the base with itself, its substituent, water, and Na+ ions. Although the sum of these components was nearly constant over a wide range of solutes the individual free energy constituents varied significantly. Furthermore, this decomposition showed a high degree of additivity. Computational conditions necessary for obtaining additive free energy decomposition for the systems studied here within the framework of the FEP method included the use of a single mutation pathway and a subdivision of the FEP protocol into 51 or more windows.