Substituent effects on the formation of cesium cation complexes with a series of 17 benzoic acids (AH), benzoates (A–), and the ion pairs (Cs+A–) are studied by density functional theory (DFT) and mass spectrometry. This study is positioned in the context of the fate of cesium in the environment, with emphasis of the influence of natural organic matter and humic substances. The bond length Cs+-(carboxylic O) in the various adduct geometries are discussed as regards the interaction strength, but quantitative relationships are limited by secondary effects arising mostly from long-distance interactions in systems bearing polar groups in meta-position. Relative cesium cation affinities of [Cs+A–] were experimentally determined by the kinetic method, i.e. by dissociating the required cesium cluster formed by electrospray ionization in a quadrupole ion-trap. Experiments and calculations are in agreement, except for the adducts derived from 3- and 4-hydroxybenzoic acids. A change in the localization of the negative charge is proposed as a possible explanation for the divergence.