Average particle diameters and electrophoretic mobilites of poly(N-isopropylacrylamide) latex were measured as a function of temperature. Diameters decreased from 788 nm at 10 °C to 380 nm at 50 °C in 0.001 M KC1; the corresponding electrophoretic mobilities increased from -0.193 X 10""8 (18 °C) to -3.06 x 10*8 m2 V'1 s'1 (47 °C). The most dramatic changes with temperature occurred around 31 °C, the lower critical solution temperature of poly(TV-isopropylacrylamide) in water. The increased electrophoretic mobility with temperature reflected increasing charge density when the particle diameter decreased. Charge density increased with decreasing particle diameter because the number of charged groups per particle was constant.