We present new measurements of the partitioning of the polymers (poly(ethylene glycol) (PEG) and poly(ethylene imine) (PEI)) and the protein bovine serum albumin (BSA) between coexisting liquid phases of isobutyric acid + water. We show that all partition unevenly between the liquid phases: In all cases, there is substantially more polymer or protein in the upper, isobutyric acid-rich phase. At molecular masses above about 10 kg/mol, PEG and PEI fractionate between the liquid phases, with a higher average molecular mass in the lower, water-rich phase. The fractionation and partitioning are amplified both when the molecular mass of the polymer is increased, and when D2O + isobutyric acid is used instead of H2O + isobutyric acid. The dependence of the distribution coefficient oil molecular mass disagrees with the predictions of Flory-Huggins mean field theory for all PEG and PEI systems studied. At low molecular weight, the PEG fractionations are consistent with theoretical predictions that the molecular mass distributions of the daughter phases will have the same mathematical forms as the parent phase. However, as the average molecular mass increases, the PEG molecular mass distributions of the daughter phases differ from those of the parent phases. The critical temperature, T-c, of PEG + isobutyric acid + H2O increases approximately linearly with PEG concentration, increases nonlinearly with PEG average molecular mass, and is independent of the terminating group.