Temperature-responsive copolymeric microsized hydrogels were prepared by the free-radical polymerization of N-isopropylacrylamide (NIPAM) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) in the presence of polyethylene glycol (PEG) as the macroinitiator. The thermosensitivity and swelling behavior of these microgels were studied by dynamic light scattering (DLS). A hydrogel prepared without AMPS (i.e., PNIPAM homopolymer) exhibited a lower critical solution temperature (LCST) that was close to room temperature, but the LCST shifted when the PNIPAM was copolymerized with AMPS. The volume phase transition temperatures of the copolymer microgels were determined from the derivative of the hydrodynamic diameter with respect to temperature $$ \left( {\frac{{{\text{d}}{D_{\text{h}}}}}{{\text{d}}T }} \right), $$ and this parameter was found to increase with increasing AMPS content in the feed recipe. During the heating process, the microgel particles dispersed in water presented a broad bimodal size distribution at 25 °C and a narrow monomodal size distribution at 50 °C. This indicates that the PNIPAM chains in the network structure shrink and the hydrodynamic diameter of the microgel decreases with increasing temperature. The hydrodynamic diameters of the microgels were found to range from 132.5 nm to 538 nm. The swelling ratio and thermosensitive properties of the microgels increased with decreasing AMPS content. The decay rate (Γ) and the diffusion coefficient (D) were found to decrease with increasing AMPS concentration in the feed. In addition, the shapes and sizes of the microgel particles were examined by transmission electron microscopy (TEM).