The structure of TMA-Ac 2 m in aqueous solutions was probed using neutron scattering at the 7C2 diffractometer, employing the technique of neutron diffraction with isotopic substitution (NDIS). The double difference signal ∆∆S H TMA H Ac (Q), known as the second order difference (see Equation and Figure ), reports on the single correlation between TMA and acetate non-exchangeable hydrogens, which makes it a very sensitive probe of the formation of TMA-Ac ion pairs and their structure. The Fourier-transformation of the experimental signal to real-space ∆∆G H TMA H Ac (r) is the single radial distribution function between H TMA and H Ac :