The phase diagrams of various ethylene-vinyl alcohol (EVAL) copolymer with different hydroxyl (OH) group contents and poly(ethylene glycol) (PEG) with different molecular weights (ranging from 200 to 600) are determined. It is found experimentally that both the liquid-liquid (L-L) phase boundaries and the crystallization curves are shifted to higher temperature when the OH group contents in EVAL increase. On the other hand, only the L-L phase boundaries shift to higher temperature when the molecular weights of PEG increase. These phenomena are interpreted by using solubility parameters (δd, δp) to estimate the Flory-Huggins interaction parameters, χ*, of the mixtures. By extrapolating the L-L bi-phasic curves from cloud points of temperature (Tcloud), the experimental interaction parameters χ* are obtained, which are linearly correlated to the theoretically estimated interaction parameters, χ*.