A series of polyethylenimine (PEI)-based water-soluble polymers (WSPs) were prepared by attaching functional groups (beta-diketones, carboxylic acid, salicylic acids) to the polymer backbone, with the goal of characterizing the interaction between beryllium and the various polymers. The extraction of beryllium from aqueous solutions by the WSPs was examined as a function of pH and ionic strength to evaluate the potential for the WSPs to isolate beryllium from contaminated aqueous waste streams. The loading capacities of these polymers for beryllium at near-neutral pH were unusually high in the absence of ionic strength adjustment compared to that for other +2 cations, suggesting that polynuclear beryllium species were interacting with the polymers. Beryllium loading capacity values were similar for all polymers evaluated in the absence of ionic strength adjustment. However, when the ionic strength of the solutions was adjusted to 0.1 N (NaNO3) the loading capacities were significantly reduced, indicating that electrostatic attraction played a dominant role in the interaction between beryllium and the polymers. The extraction curves of beryllium for all polymers evaluated, even those not designed to be selective for beryllium extraction, were nearly identical irrespective of the nature of the functional groups. Collectively, these results suggest that oligomeric beryllium species were formed, which can bind to the polymers through a combination of electrostatic forces and, potentially, hydrogen bonding.