F-19 NMR chemical shifts have been measured to determine the critical micelle concentration (cmc) and aggregation number (m) of lithium perfluorooctylsulfonate (LiFOS) in water at temperatures ranging from 30 to 250 degrees C under the saturation pressure. The cmc increases moderately in the temperature range from 30 to 100 degrees C, whereas above similar to 150 degrees C the increase is steep; cmc is 6.5, 13.2, and 161 mmol kg(-1) at 30, 100, and 250 degrees C, respectively. The aggregation number, estimated on the basis of a mass action model, exhibits a rapid decrease in the temperature range of 30-150 degrees C and becomes almost constant at higher temperatures; m is 36, 6.8, and 2.4 at 30, 100, and 250 degrees C, respectively. Thermodynamic parameters obtained indicate that the micellization is entropy driven at ambient temperatures and energy driven at high temperatures above 150 degrees C.