Ultrasonic velocities and densities of binary mixtures of acetonitrile (AN) and n-butyronitrile (n-BTN) with Bu4NBPh4, Bu4 NClO4, Bu4 NI, Bu4NBr, Pr4NBr, Et4NI, Et4NBPh4, NaBPh4, NaClO4 and CuClO4 have been measured in the concentration range 0.0045 - 0.2 mol kg−1 over the entire composition range at 298.15 K. Isentropic compressibilities (Ks) and apparent molal isentropic compressibilities (Ks,ø ) have been calculated. Limiting apparent molal isentropic compressibilities (Kos,ø ) have been evaluated and split into the contribution of individual ions, i. e. into (Kos,ø )± values. (Kos,ø )± for Cu+, Na+ and Br− in all cases is negative and large, for Bu4N+ and Ph4B− positive and large, and for I− and ClO4− negative only in AN, and becomes positive in binary mixtures of AN and n-BTN for all compositions. The negative (Kos,ø )± values for Cu+, Na+ and Br− indicate strong ionsolvent interactions involving electrostatic ion-dipole interactions. The large and positive (Kos,ø )± values for Bu4N+ and Ph4B− show some special type of ion-solvent interactions (hydrophobic or dispersion interactions) with AN and n-BTN. The small negative value changing to a small positive value for I− and ClO4 − indicates the change of a weak electrostatic ion-solvent interaction to another weak special interaction. A comparison of (Kos,ø )± for Cu+ in some solvent systems shows that the ion-solvent interaction behaviour of Cu+ in AN+ n-BTN is similar to that in AN+ NM, AN+ TEP, BN+ TEP and PY + TEP mixtures but differs from that observed in AN+DMF and AN+DMSO mixtures.