A thin film balance of Scheludko-Exerowa type was used to determine equilibrium film thicknesses of dodecylammonium chloride (RNH3Cl) solutions. The data were analyzed in view of the extended DLVO theory, which considers electrostatic, van der Waals, and hydrophobic forces. The hydrophobic force was represented as a power law which is of the same form as for the van der Waals force, so that its constant K-232 can be directly compared with the Hamaker constant, A(232). The results showed that at low surfactant concentrations, K-232 is positive and decreases with increasing surfactant concentration, suggesting that hydrophobic force plays an important role in thin films. When the K-232 versus concentration plot was extrapolated to very dilute solutions, K-232 approaches 10(-17) J, which is approximately 270 times larger than A(232). When the surfactant concentration was increased above 2 x 10(-3) M, however, K-232 becomes negative, indicating that hydration force appears at high surfactant concentrations. These results suggest that air bubbles are hydrophobic and the hydrophobicity decreases with increasing surfactant concentration. A TFB was used to obtain a disjoining pressure isotherm at 10(-3) M RNH3Cl in the presence of 10(-4) M NaCl. The results can be fitted to the extended DLVO theory with K-232 = 6 x 10(-19) J. Consideration of hydrophobic force predicted a rupture thickness larger than predicted using the DLVO theory, but is substantially smaller than the experimental result. This discrepancy may be ascribed to the hydrodynamic force operating in the film thinning process,