The pertechnetate anion quantitatively partitions to the upper polyethylene glycol (PEG)-rich phase in aqueous biphasic systems (ABS) formed by the addition of aqueous high-molecular-weigh PEG solutions to aqueous water-structuring salt solutions. Most metal cations partition to the salt-rich phase in these systems. Other matrix ions, such as those that might be present in real-world waste streams, affect the distribution ratios observed for TcO4-, however, the effects are not well understood. The halide ions F-, Cl-, Br-, I- represent a wide range of water-structuring to chaotropic ions. The heavier halide ions have distribution ratios above one and are observed to follow the order D-I > D-Br > D-Cl. These ions depress D-Tc. Fluoride salts-out PEG and thus partitions primarily to the salt-rich phase and fluoride increases D-Tc. These trends can be correlated with the ions’ Gibbs free energies of hydration (Delta G(hyd)), a measure of their interactions with water. Based upon these results, it is possible to predict which ions will depress, elevate, or not affect D-Tc for a given PEG-ABS based upon the matrix ion’s Delta G(hyd).