The temperature- and pH-sensitive copolymers of N-isopropylacrylamide (NIPAAm) with 4-pentenoic acid (PAc) were synthesized and the monomer reactivity ratios were estimated. The effects of polymer composition, pH and ionic strength of the aqueous solution on the lower critical solution temperature (LCST) of the copolymers were investigated. It was found that the LCSTs of these unusual copolymers can be either lowered or raised compared to the LCST of poly(N-isopropylacrylamide) by changing the pH.