The use of molecular solvents has been proposed as a simple solution to reduce the high viscosities of neat ionic liquids (ILs) and extend the practical applications of ILs. A proper understanding of the structure and intermolecular interaction is of vital importance for the design, optimization and synthesis of ILs systems with tailored properties for specific applications. In this work, the structure and hydrogen-bond features of the representative pyrrolidinium/piperidinium based ILs and acetonitrile mixtures were studied by a combination of Fouriertransform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations. The nu(C N) region is sensitive to the microenvironment and is used as IR probe to detect the structure and hydrogen-bond properties of the two IL-acetonitrile binary systems in the whole concentration range. Positive peaks are observed in the excess IR spectra of nu(C N) region which indicates the non-ideality of the mixing process and the formation of hydrogen-bonded complexes in the mixtures. With the help of deconvolution and DFT calculations, the species transformation in the mixing process can be identified from the excess spectra: When x(CD3CN) is less than 0.90, acetonitrile mainly interacts with the ion pairs and ion clusters of the ILs. Ion clusters are all broken out into ion pairs and the interaction complex is mainly ion pair-CD3CN when x(CD3CN) > 0.90. In the whole concentration range, the CD3CN cannot break apart the strong coulombic interactions between the cation and anion, and the individual cation and anion do not exist in the mixtures. All of the hydrogen-bonds in the ion pair/ion clusteracetonitrile complexes are weak strength, closed shells and electrostatically dominant interactions. (C) 2020 Published by Elsevier B.V.