Alkali metal cations often show pronounced ion-specific interactions and selectivity with macromolecules in biological processes, colloids, and interfacial sciences, but a fundamental understanding about the underlying microscopic mechanism is still very limited. Here we report a direct probe of interactions between alkali metal cations (M+) and dicarboxylate dianions, (-OC)-C-2(CH2)(n)CO2- (D-n(2-)) in the gas phase by combined photoelectron spectroscopy (PES) and ab initio electronic structure calculations on nine M+-D-n(2-) complexes (M = Li, Na, K; n = 2, 4, 6). PES spectra show that the electron binding energy (EBE) decreases from Li+ to Na+ to K+ for complexes of M+-D-2(2-), whereas the order is Li+ < Na+ approximate to K+ when M+ interacts with a more flexible D-6(2-) dianion. Theoretical modeling suggests that prefers to interact with both ends of the carboxylate -COO- groups by bending the flexible aliphatic backbone, and the local binding environments are found to depend upon backbone length n, carboxylate orientation, and the specific cation M+ The observed variance of EBEs reflects how well each specific dicarboxylate dianion accommodates each M+. This work demonstrates the delicate interplay among several factors (electrostatic interaction, size matching, and strain energy) that play critical roles in determining the structures and energetics of gaseous clusters as well as ion specificity and selectivity in solutions and biological systems.