The binding of K+ and Ba2+ cations to short poly(ethylene oxide) (PEO) chains with ca. 4-25 monomeric units in methanol was studied by determining the effective charge of the polymer through a combination of electrophoretic NMR and diffusion NMR experiments. These cations were previously found to bind to long PEO chains in a similar strong manner. In addition, H-1 chemical shift and longitudinal spin relaxation rate changes upon binding were quantified. For both systems, binding was stronger for the short chains than that for the longer chains, which is attributed mainly to interactions between bound ions. For K+ ions, the equilibrium binding constant of a cation to a binding site was measured. For both cations, the binding site was estimated to consist of ca. six monomeric units that coordinated with the respective ions. For the systems with barium, a significant fraction of the bound ions are (BaAnion)(+) ion pairs. This leads to a strong anion effect in the effective charge of the oligomers acquired upon barium ion binding. For K+, the coordinating oligomer segment remains rather mobile and individual oligomers exchange rapidly (<s) between their free and barium complexing states.