The densities have been determined for the binary systems of ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate, [Emim][BF4] and 1-hexyl-3-methylimidazolium tetrafluoroborate, [Hmim] [BF4], with the molecular solvents dimethyl sulfoxide (DMSO) and acetonitrile (ACN) at atmospheric pressure over the entire range of composition and temperatures from (298.15 to 353.15) K. From experimental densities, the excess molar volumes, V-E, were calculated and fitted to a Redlich-Kister equation. Moreover, the V-E results have been discussed in terms of the Prigogine-Flory-Patterson (PFP) theory. The V-E variation was interpreted in terms of intermolecular interactions and structural effects on mixing.