In this work, the effect of cononsolvency on the phase transition and preferential adsorption phenomenon behaviors of poly(N-isopropylacrylamide)/methanol/ water ternary solutions was studied. In this cononsolvent system, the DV E , DG m , and v 12 values show a nonlinear behavior and the minimum values of DV E and DG m , while the maximum value of v 12 at / 2 is around 0.7. These facts indicated that one water molecule could directly bond with one methanol molecule to form the H 2 OÀ ÀMeOH complex. The H 2 OÀ ÀMeOH complex structure was found to remarkably affect the phase transition of poly(N-isopropylacrylamide) (PNIPAM) in ternary solution. However, at the composition of mixed cononsolvent, / 2 < 0.2, the PNIPAM molecules may preferentially adsorbed pure water molecules; therefore, the LCST decreases slightly with composition of mixed cosolvent and this may be because of the small amount of H 2 OÀ ÀMeOH complexes in the mixed cononsolvent. While, at / 2 > 0.7, the PNIPAM molecules may preferentially adsorbed pure methanol molecules. PNIPAM ternary solutions were transparent and no transition occurred in this region. This indicates that the PNIPAM coils exhibited a much-extended conformation in solutions. In contrast, at 0.2 < / 2 < 0.4 and 0.4 < / 2 < 0.7, PNIPAM molecules preferentially adsorbed water and methanol molecules, respectively, and also adsorbed large amount of H 2 OÀ ÀMeOH complexes. In these regions, the clathrate-like structure around the side chain of PNIPAM molecule became more defected with adsorbing H 2 OÀ ÀMeOH complex. Therefore, we considered that the various thermodynamic behaviors between PNIPAM and mixture solvents must be related different preferential adsorption phenomena, which were mainly related to different degrees of polymer-solvent interaction and structures of solvent used.