[en] Highlights: • XPS is used to investigate trioctylmethylammonium ionic liquids. • The cation-anion interaction is revealed in detail. • The impact of both the cation and the anion on the charge-transfer effect is explored. • The anion can be effectively tuned by using ionic liquid mixtures with binary cations. - Abstract: In this study, two trioctylmethylammonium ionic liquids were investigated using X-ray photoelectron spectroscopy, together with two binary ionic liquid mixtures. The interaction between the cation and the anion is demonstrated in detail. The effect of both the anion basicity and the cation acidity on the charge transferred from the anion to the cation is explored. By using ionic liquid mixtures with binary cations, the electronic environment of the anion can be effectively tuned. XPS data can be used to reflect such subtle change.