Blends of poly(acrylic acid), PAA, and poly(ethylene glycol), PEG, are miscible over the entire range of composition as a result of hydrogen-bonding between the acids and ether oxygens. Infrared analysis showed that the formation of acid-ether complexes is preferred over that of cyclic acid dimers formed from self-association of the carboxylic acid groups. Miscibility decreased as the extent of neutralization of acrylic acid (AA) to the lithium salt (LiA) increased, and lower critical solution temperatures (LCST) were observed for blends made with AA-LiA copolymers containing 0.1-0.4 mole fraction LiA. Pairwise interaction energies between the different chemical species were calculated from equilibrium melting point depression data and a binary interaction model. Favorable interactions between AA and ethylene glycol (EG) promoted miscibility, while an unfavorable interaction between LiA and EG favored demixing. Infrared analysis showed that neutralization of the acid reduced the concentration of acid-ether complexes and promoted formation of acid cyclic dimers that favored phase separation.