Monodisperse poly(N-isopropylacrylamide-co-acrylic acid) hydrogel microspheres were prepared by a membrane emulsification method using membranes of pore diameters of 0.33, 0.73, 1.15, and 1.70 mu m. The hydrogels were synthesized by polymerization of 3.6 M N-isopropylacrylamide (N-IPAAm or NIPAM) and 0.4M acrylic acid (AAc). Their surface properties were studied by measuring the electrophoretic mobility of the microspheres in electrolyte solutions at pH 7.4 at 25, 30, 33, 35, 40, and 45 degrees C. Poly(N-IPAAm-co-AAc) microspheres have shown negative mobility. More negative values of electrophoretic mobility were obtained with the smaller microspheres than the larger ones at each temperature. The surface charge density of the microspheres increased and their surfaces became harder above 35 degrees C, since the microspheres contained thermosensitive poly(N-IPAAm) moiety and LCST increased by the addition of AAc, while that of poly(N-IPAAm) was 33 degrees C. It has recently been found that the smaller microspheres exhibit the stronger dependence of both surface charge density and softness on the temperature.