The solvatochromic properties α, β, and π* have been determined for an aqueous biphasic system (ABS) composed of polyethylene glycol and the salts (NH₄)₂SO₄ or K₃PO₄ using the probes Reichardt's carboxylated betaine, 4-nitrophenol, and 4-nitroanisole. It was found that the co-existing phases rich in either polymer or salt differed little in their relative polarity or relative basicity and that their basicity and polarity differed little from that of water. On the other hand, the polymer-rich phase was found to be significantly depleted in H-bond donor ability relative to the salt-rich phase and to pure water. The results of this study compared favorably with the results of a solute distribution study encapsulated in a linear free energy relationship. As a result, it seems possible to conclude that the principal molecular determinants of partitioning in ABS are the solute properties size, basicity, and aromaticity or halogenicity.