A series of pH-and temperature-responsive (N,N-diethylacrylamide-co-methacrylic acid) (DEAM-MAA) random copolymers were synthesized by free-radical copolymerization techniques, and their lower critical solution temperatures (LCSTs) at different pH values and their critical phase-transition pH values at different temperatures were determined by transmittance measurements, respectively. The LCSTs of DEAM-MAA copolymers depend on the MAA content in the copolymers and are significantly affected by the pH of the aqueous solution. Similarly, the critical phase-transition pH values also depend on the MAA content of the DEAM-MAA copolymers and are significantly affected by the temperature of the aqueous solution. The LCST of DEAM-MAA copolymers increases with an increase in the MAA content. The critical phase-transition pH increases with increasing MAA content in the copolymer and also increases with increasing aqueous solution temperature.