Quasielastic neutron scattering (QENS) spectra were measured for a 0.43 M solution of n-tetrapentylammonium bromide in deuterated acetonitrile at 25 °C, 5 °C, and −15 °C. Values of the translational diffusion coefficient of the cations were inferred from these data using a simple model of translation and rotation. These values are significantly higher than the ones obtained by nuclear magnetic resonance (NMR) spin–echo measurements. The difference can be explained by the different time scales covered by QENS and NMR. QENS shows essentially the contribution of a second order electrophoretic effect to the diffusion coefficient whereas NMR encompasses both electrophoretic and relaxation effects. Consequently, the combination of both techniques allows the two effects to be separated. The relaxation contribution to the diffusion coefficient was calculated by brownian dynamics simulation and compared to the experimental results. The solvent-averaged ion pair potentials used for this computation were simultaneously adjusted to the thermodynamic and to the small-angle neutron scattering data by means of hypernetted chain (HNC) calculations.