The solute−solvent interaction dictates the equilibrium structure of ionic assemblies in solution, ranging from free ions, ion pairs, ion-pair dimers, and ion-pair tetramers to nanoparticles and ionic crystals. We use two-dimensional infrared (2DIR) spectroscopic measurements of the antisymmetric CN stretch of thiocyanate (NCS−) to study the equilibrium chemical exchange between the spectrally distinct ion pair and the ion-pair dimer in benzonitrile. We observe this chemical exchange to occur with a 36 ± 4 ps time constant. Our measurement indicates that 2DIR will provide a useful tool for investigating the dynamics of ion assembly in solution.Keywords (keywords): 2DIR; LiNCS; ionic assemblies; chemical exchange; dimer