Critical micelle concentrations of AOT in water in the presence of sodium chloride, sodium acetate, sodium propionate, and sodium butyrate were determined at 25 degrees C by the surface tension method. The co-ions do not have any effect on the value of critical micelle concentration. The surface density of AOT at the air-water interface increases in the presence of added electrolyte and attains a maximum value of 2.5 +/- 0.1 mol m(-2) at a particular electrolyte concentration which is different for sodium chloride and the other three electrolytes. From the Corrin-Harkins plot it has been found that for AOT micelles the counterion binding constant has values 0.40 and 0.82 below and above similar to\0.015 mol kg(-1) electrolyte concentration (c*), respectively. Measurement of sodium ion activity from the EMF method has confirmed such a shift in the counterion binding constant of AOT at c*. The higher value of the counterion binding constant for AOT has been reported for the first time. From fluorescence spectroscopy it has been found that the aggregation number of AOT is 22 in water and its average aggregation numbers in the presence of electrolytes are about 34 and 136 below and above c*, respectively. The increase by a factor of 2 in the counterion binding constant is shown to be due to a change in the shape of the AOT micelles around c*. The shape of AOT micelles in the electrolyte concentration range <= c* is inferred to be oblate spheroid and a change from this shape appears to occur above c*. A sudden increase in the polarity of the micelle-solution interface is also observed above c*. (c) 2005 Elsevier Inc. All rights reserved.