A structural and vibrational study on the antiviral deoxyribonucleoside thymidine in gas and aqueous solution phases was performed combining the available infrared and Raman spectra with density functional theory (DFT) calculations. Three stable conformers for the title molecule were theoretically determined in both media by using the hybrid B3LYP method together with the 6–31G* basis set. The solvent effects were studied by means of the self-consistent reaction field method employing the polarized continuum model. Complete assignments of the vibrational spectra of thymidine in both phases were performed combining the DFT calculations with Pulay's scaled quantum mechanics force field methodology. The bond orders, atomic charges, solvation energies, dipole moments, molecular electrostatic potentials, and force constants parameters were calculated for the three conformers of thymidine in gas phase and aqueous solution. Selected descriptors, such as chemical potential (µ), electronegativity (χ), global hardness (η), global softness (S), and global electrophilicity index (ω) were used to explain the exact nature of the interactions with electrophiles and/or nucleophiles and to predict the behavior of the three conformers of thymidine in gas and aqueous solution phases. © 2013 Wiley Periodicals, Inc.