The effects of the monomer ratio, surfactant, and crosslinker contents on the particle size and phase-transition behavior of the copolymer poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAAm-MAA) were investigated with Fourier transform infrared, differential scanning calorimetry, and dynamic laser scattering techniques. In addition to the thermoresponsive property of poly(N-isopropylacrylamide), ionized methacrylic acid groups brought pH sensitivity to the PNIPAAm-MAA copolymer particles. The polymer particle size varied with the amounts of the monomer ratio, surfactant, and crosslinker. As the monomer ratio and crosslinker content increased and the amount of the surfactants decreased, the particle size increased. The influence of the crosslinker content on the particle size was less significant than the effect of the monomer ratio and surfactants. When the temperature increased, the particles tended to shrink and decreased in size to near or below 100 nm. Particle sizes at 20 degrees C decreased to less than 100 nm with increased surfactant content. The control of the particle size within the 100-nm range makes PNIPAAm-MAA copolymer particles useful for biomedical and heavy-metal-ion adsorption applications. (C) 2008 Wiley Periodicals, Inc.