Abstract Structural specificity including dynamic behavior and stable conformations in the bulk phase of diethylmethylammonium trifluoromethanesulfonate [dema][TfOH], which is assumed to be a prominent ionic liquid electrolyte for non-humidified intermediate temperature fuel cells, have been investigated by 1H NMR, IR spectroscopic analyses, and molecular dynamics (MD) simulation. It is found that an N–H proton in [dema][TfOH] is an exchangeable mobile proton which can be substituted by D2O, and free rotation around the N–C bond in the ethyl side chain of the ammonium cation is retarded by ionic interactions between the cations and anions, therefore the methylene protons in the ethyl side chain are unisochronous. In addition, the rotational barrier of this N–C bond was observed to be 71 kJ mol−1, comparable to the barrier height of the amide N–C bond (which is well-known to have partial double bond character) from temperature-dependent NMR experiments by monitoring the peak-shape changes of methylene protons in the ethyl side chain of the ammonium cation. The bulk-phase structure of [dema][TfOH] was calculated by MD simulations on the basis of the OPLS-AA force field, and the evaluated structure was consistent with those of experimental results. Thus, IR spectrum frequency of the N–H proton, and the 1H NMR chemical shift values could be rationally assigned on the basis of the theoretically evaluated structures in the bulk phase.