The copolymers of N-isopropylacrylamide and methacrylic acid (P(NIPAM-co-MAA)), and the copolymers of N-isopropylacrylamide, methacrylic acid, and octadecyl acrylate (P(NIPAM-co-MAA-co-ODA)) were synthesized by a free radical reaction. MAA was copolymerized as a pH-sensitive residue and ODA as a hydrophobic moiety. The transmittances of P(NIPAM-co-MAA) solutions were almost constant in the pH 2.0-8.5. On the other hand, the transmittances of P(NIPAM-co-MAA-co-ODA) solutions were reduced in the pH 2-5, whereas the values were almost invariable in the pH 5.0-8.5. In the low range of pH, most of carboxyl groups are in unionized forms. Therefore, the hydrophobic interactions among ODAs might be greater than the electrostatic repulsions. Self-assemblies such as polymeric micelles could be formed, leading to the reduced transmittances. In the measurement of air/water interfacial tensions, the surface activities of both P(NIPAM-co-MAA) and P(NIPAM-co-MAA-co-ODA) were lower in alkali pHs than in acidic conditions. This is possibly because that the copolymers are more hydrophilic at higher pHs due to the ionization of the carboxyl groups. P(NIPAM-co-MAA-co-ODA) was more surface-active than P(NIPAM-co-MAA) in the full range pH, pH 2.0-8.5, due to the hydrophobic effect of ODA. (C) 2008 Wiley Periodicals, Inc.