The surface tension of colloidal poly(N-isopropylacrylamide) (PNIPAM) dispersions was measured as functions of surface age, temperature, and the morphology of the PNIPAM microgels. The microgels lowered the surface tension of water to about 43 mJ/m(2) at 25 degrees C and a little less at 40 degrees C. The steady-state surface tension values were not very sensitive to temperature or to the degree of microgel cross-linking. On the other hand, the time required to reach a steady state was dependent upon microgel morphology. The lower the cross-linking or the lower the particle uniformity, the more rapid was the surface tension decline. Microgels were observed to form an ordered array at the air/water interface when viewed by an environmental scanning electron microscope. It was proposed that the rate of surface tension lowering was in part influenced by the rate of particle spreading after adsorption onto the interface.