Molecular dynamics computer simulations have been performed for Na+-water-formamide mixtures over the whole concentration range of the mixed solvent. It is shown that the Na+ cation is preferentially solvated by formamide even if only small amounts of formamide are present in the mixture. Specific cation solvent shell formation leads to a drastic change of the ion microdynamics in the mixture compared to pure solvents. A possibility of extension of the molecular theory of limiting ionic mobility to mixed solvents was tested and could not be confirmed by the simulation. (C) 1997 American Institute of Physics. [S0021-9606(97)52041-1].