The surface tension-concentration (gamma-c) isotherms of soluble surfactants, unlike those of insoluble surfactants, are not sufficient to show whether gas-liquid-phase transitions occur. However, thermodynamic parameters (such as thermal coefficients) can be used to prove the existence of such transitions. The surface thermal coefficient corresponding to (partial derivativegamma/partial derivative ln T)(A,P) was described by Defay and named b(TP). We determined this surface coefficient for a nonionic surfactant, Triton X100 (TX100), both in water and in a salt solution. We used the Wilhelmy plate method to measure the surface tension of TX100 at between 279 and 303 K. The b(TP) = f (log [TX100]) plot showed a clear peak, which could be attributed to a surface gas-liquid transition. Temperature and salt concentrations did not affect this transition.