Solvent effects on the C-N bond rotation process of formamide (FA) are investigated experimentally and theoretically. Temperature-dependent exchange broadened H-1 NMR line shapes of [N-15]FA in water, dimethyl sulfoxide-d(6), and tetrachloroethane-d(2), and as neat solution are consistent with Delta G double dagger(298) values for FA internal rotation, in kilocalories per mole, of 18.2(0.1), 17.8(0.1), 17.2(0.1), and 18.0(0.1), respectively. These values are significantly higher than the gas-phase Delta G double dagger(298) value, 16.0(0.1) kcal mol(-1), and they correlate well with empirical spectroscopic solvent polarity scales. Self-consistent isodensity polarizable continuum model (SC-IPCM) calculations at the HF/6-311++G** level of theory for FA in a series of dielectric constants ranging from 1 to 109 predict a complete reversal of the preferred internal rotation path for hydrated FA compared to gas-phase FA. Equal contributions to the internal rotation rate constant from both possible internal rotation paths is predicted to occur at a dielectric constant of 7.08. SC-IPCM calculations predict that Delta G double dagger(298) for hydrated FA is 1.99 kcal mol(-1) higher than Delta G double dagger(298) for gas-phase FA. SC-IPCM calculations for a 1:1 FA-H2O complex were performed to allow for direct inclusion of intermolecular hydrogen bonding. The predicted Delta G double dagger(298) for the hydrated 1:1 FA-H2O complex is ca. 2.3 kcal mol(-1) higher than the predicted gas-phase Delta G double dagger(298), in excellent agreement with experiment.