Precise measurements on electrical conductances of sodium tetraphenyl borate (NaPh4B) and tetrabutyl ammonium tetraphenyl borate (Bu4NPh4B) in different mass fraction (0.00 to 1.00) of nitromethane (CH3NO2) in nitrobenzene (PhNO2) at 298.15 K have been performed. Limiting molar conductances (Lambda(0)), association constants (K-A) and co-sphere diameter (R) for ion-pair formation in the mixed solvent systems have been evaluated using the Fuoss conductance-concentration equation. The FT-IR spectra for the solvents as well as the solute in solvent have also been studied. The results show more association of the ions in PhNO2 than CH3NO2 and results have been interpreted in terms of ion-solvent and solvent-solvent interaction. (C) 2012 Elsevier Ltd. All rights reserved.