Combining two ionic liquids to form a binary ionic liquid mixture is a simple yet effective strategy to not only expand the number of ionic liquids but also precisely control various physicochemical properties of resultant ionic liquid mixtures. From a fundamental thermodynamic point of view, it is not entirely clear whether such mixtures can be classified as ideal solutions. Given a large number of binary ionic liquid mixtures that emerge, the ability to predict the presence of nonideality in such mixtures a priori without the need for experimentation or molecular simulationbased calculations is immensely valuable for their rational design. In this research report, we demonstrate that the difference in the molar volumes (Delta V) of the pure ionic liquids and the difference in the hydrogen-bonding ability of anions (Delta beta) are the primary determinants of nonideal behavior of binary ionic liquid mixtures containing a common cation and two anions. Our conclusion is derived from a comparison of microscopic structural properties expressed in terms of radial, spatial, and angular distributions for binary mixtures and those of the corresponding pure ionic liquids. Molecular dynamics simulations of 16 binary ionic liquid mixtures, containing a common cation 1-n-butyl-3-methylimidazolium [C(4)mim](+) and combinations of (less basic) fluorinated {trifluoromethylacetate [TFA](-), trifluoromethanesulfonate [TFS](-), bis(trifluoromethanesulfonyl) imide [NTf2](-), and tris (pentafluoroethyl) trifluorophosphate [eFAP](-)} versus (more basic) nonfluorinated {chloride Cl-, acetate [OAC](-), methylsulfate [MeSO4](-), and dimethylphosphate [Me2PO4](-)} anions, were conducted. The large number of binary ionic liquid mixtures examined here enabled us to span a broad range of Delta V and Delta beta values. The results indicate that binary mixtures of two ionic liquids for which Delta V > 60 cm(3)/mol and Delta beta > 0.4 are expected to be microscopically nonideal. On the other hand, Delta V < 60 cm(3)/mol and Delta beta < 0.4 will lead to molecular structures that are not differentiated from those of their pure ionic liquid counterparts.