The conductivities of lithium and quaternary ammonium salts have been measured in propylene carbonate and gamma-butyrolactone at 25 degrees C over the concentration range of 10(-2) to 10(-3) mol dm(-3). The limiting molar conductivities and the ionic association constants were calculated by the expanded Fuoss-Hsia equation. The mobility of anions in both solvents decreased in the following order : BF4- > ClO4- > PF6- > AsF6- > CF3SO3- > (CF3SO2N- > C4F0SO3- > BPh(4)(-) The association constants increased in the following order : Li(CF3SO2)(2)N, LiAsF6 < LiPF6 < LiClO4 < LiBF4 < LiC4F0SO3 < LiCF3SO3, while those of the corresponding tetraethylammonium salts were leveled by electrostatic shielding effect of alkyl groups. These results were correlated with ionic radii obtained by MM2 calculation.