The solution microstructure of aqueous (D2O) poly(ethylene glycol) (PEG) solutions was determined as a function of polymer concentration and molecular weight in the presence of salts such as NaNO3, Na2CO3, and Na3PO4. Measurements were made by using small-angle neutron scattering (SANS). In addition, anion effects on biphase formation were investigated by cloud point measurements in H2O and D2O. In the presence of NaNO3, PEG behaves like a random coil with no evidence of aggregation, whereas the presence of either Na3PO4 or Na2CO3 leads to aggregate formation. Furthermore, the aggregate size increased with increasing salt concentration until a discontinuity appeared at the point of biphase formation, where the polymers in the PEG-rich phase form an entangled mesh with loss of chain identity. In the monophasic regime, the aggregates were elongated with a radius of approximately 19 Angstrom and a length which varied with salt type and concentration. Analysis of the effects of electrolytes on the cloud points of PEG in H2O and D2O suggests that its phase behavior in solution is entropy driven. Salt effects on cloud points follow the well-known Hofmeister series. Anions leading to increased structuring of water lower the cloud point, as does the substitution of D2O for H2O. The cloud point data correlate linearly with the change in water entropy upon the addition of electrolytes.