The effects of the addition of hydrophobic substances on the volume-phase transition of N-isopropylacrylamide (NIPA) hydro gels were investigated. The volume transition temperature of NIPA gel was reduced by adding hydrophobic substances, such as benzoic acid (BA) and phenol (Ph), into the solution surrounding the gel. The degree of binding of BA or Ph to the NIPA gel was found to increase in a transition-like manner simultaneously with discontinuous volume shrinkage. The lowering of the transition temperature relates to the degree of binding of BA and Ph to the NIPA gel in the collapsed state. Fourier transform infrared (FTIR) spectra of the Ph binding gel suggest that the hydrogen bond forms between the amide group of the NIPA and the hydroxyl group of Ph.