The role of intermolecular hydrogen bond on the stability of hydrophobically associated domains of hydrogel, consisting of 12-acryloyloxydodecanoic acid (ADA; hydrophobic) or 6-acryloyloxyhexanoic acid (AHA; hydrophobic) and acrylic acid (AA; hydrophilic), carrying an alkyl side group terminated by carboxylic acid, was studied by swelling behavior in an organic solvent/water mixture. We chose propionic acid (PAc) and 1-propanol (PrOH) as organic solvents whose solubility parameter, log P, logarithmic partition equilibrium coefficient for octanol/water, is similar. The equilibrated swelling ratio of poly(ADA-co-AA) and poly(AHA-co-AA) gels by PAc was higher than by PrOH at a lower composition (ca. 5-15 mol%) and that of homo-polymer poly(AHA) gel by PAc was higher even at a high composition (up to ca. 70 mol%). The variation in swelling with solvent/water composition indicated a cross-over between the two solvent systems and this phenomenon did not depend on the alkyl side-chain length. Using Fourier transfer infrared spectroscopy, we observed the remarkable shift of the wavelength corresponding to the hydrogen bond in PAc aqueous solutions. Non-dissociated short alkanoic acid, which can penetrate hydrophobic domains and form hydrogen bonds with non-dissociated carboxylic groups of the polymer gel, favors the disruption of hydrophobic domains, causing swelling. Intermolecular hydrogen bonds between carboxylic groups of the alkyl side-chain are thus closely involved in the stability of hydrophobic domains in copolymer gel.