The Gibbs energies of association ΔG(sol) between primary alkyl ammonium ions and crown ethers in solution are measured and calculated. Measurements are carried out by isothermal titration calorimetry (ITC) and revealed a strong solvent-dependent ion pair eﬀect. Calculations are performed with density functional theory (DFT) including Grimme’s dispersion correction D3(BJ). The translational, rotational and vibrational contributions to the Gibbs energy of association ΔG(sol) are taken into account by a rigid-rotor-harmonic-oscillator approximation with a free-rotor approximation for low lying vibrational modes. Solvation effects are taken into account by applying the continuum solvation model COSMO-RS. Our study aims at ﬁnding a suitable theoretical method for the evaluation of the host guest interaction in crown/ammonium complexes as well as the observed ion pair eﬀects. A good agreement of theory and experiment is only achieved, when solvation and the eﬀects of the counterions are explicitly taken into account.