Conductivity data for lithium picrate and lithium perchlorate in acetonitrile + nitromethane isodielectric mixtures at 25 degrees C are reported. Treatment of the data in terms of the Fuoss 1980 equation shows that the ion pair association of these electrolytes depends strongly on the composition rather than on the macroscopic dielectric constant of the solvent. Such a result has been rationalized in terms of specific ion-solvent interactions, related to the peculiar properties of the acetonitrile + nitromethane mixtures. The effect of the complexation of lithium ion with 15 crown (15C5) on the ionization of both electrolytes was also investigated. The results show that the shielding of the electric field between the lithium ion and counterion, caused by the crown ether, reduces drastically the ion pairing. The association constants for each electrolyte in the ternary system acetonitrile + nitromethane + 15C5 become, in addition, of the same order of magnitude in the whole range of solvent composition, as expected for isodielectric solvent mixtures.