Mixtures of ionic liquids (ILs) have shown their potential in both physical and chemical processes, regarded as alternatives to common ILs. In this work, four guanidinium-based ILs, 2-ethyl-1,1,3,3-tetramethylguanidinium ethyl sulfate ([TMG(C₂)][C₂OSO₃]) and bis(trifluoromethylsulfonyl)imide ([TMG(C₂)][NTf₂]), and 2,2-diethyl-1,1,3,3-tetramethylguanidinium ethyl sulfate ([TMG(C₂)₂][C₂OSO₃]) and bis(trifluoromethylsulfonyl)imide ([TMG(C₂)₂][NTf₂]), are employed to investigate the structures, interactions and properties of four systems of IL–IL binary mixtures, including [TMG(C₂)][C₂OSO₃]_x[NTf₂]_1−x, [TMG(C₂)]_x[TMG(C₂)₂]_1−x[C₂OSO₃], [TMG(C₂)]_x[TMG(C₂)₂]_1−x[NTf₂] and [TMG(C₂)₂][NTf₂]_x[C₂OSO₃]_1−x. Combining experiments with theory, the relationships among H-bond interactions, structures and volumetric properties have been revealed. ¹H NMR characterizations show the changes of H-bond interactions in the IL–IL mixtures in relation to composition, and DFT calculations reveal significant cation–anion interactions through the active hydrogen atom (N⁺–H) and the methyl groups in the cations with the anions in the manner of H⋯O and H⋯F. The ethyl group in the [C₂OSO₃]⁻ anion hardly forms interactions with other components. The size effect of the calculated system has been evaluated for the IL–IL clusters with 2, 4 and 8 ions. Different structures due to variation of cationic and anionic species have remarkable influence on the volumetric properties of the IL–IL mixtures. Negative excess molar volume (V^E_m) is found in [TMG(C₂)]_x[TMG(C₂)₂]_1−x[C₂OSO₃], and it is caused by the close packing of ions. Positive V^E_m values indicate that interaction loss occurs in the other three systems, where a linear arrangement or square packing of ions with low space utilization is found.