[en] Highlights: • A two-component fitting model for C 1s of [N_1,8,8,8]⁺ ionic liquids is developed. • The impact of the cation acidity on the cation-anion interaction is revealed by XPS. • For more basic anions, the less acidic the cation, the less charge is transferred. • For less basic anions, it concentrates more on components bearing more point charges. The impact of the cation acidity on the cation-anion interaction in ionic liquids is revealed by X-ray photoelectron spectroscopy, employing four cations with different acidity. A two-component fitting model for carbon regions of trioctylmethylammonium ionic liquids is developed. It is found that for more basic anions, the less acidic the cation, the less charge is transferred from the anion to the cation; for anions with lower basicity, such effect concentrates more on the component bearing more point charges.