The thickness of thin flat foam films formed in a range of aqueous dodecylammonium chloride solutions at pH 5.9 +/- 0.2 has been determined using a thin film balance. The results are analyzed within the DLVO theory in order to estimate the interfacial potential of the liquid-air interface. As expected, the interfacial potential increases with increasing surfactant concentration. However, it was also found that the interfacial potentials deduced from the equilibrium film thickness were significantly lower than those obtained by considering the adsorbed amount (obtained by using the Gibbs’ adsorption equation) and the pK(a)-value of dodecylamine at the interface. The interfacial potential at the air-dodecylammonium chloride solution interface was also found to be significantly lower than the corresponding interfacial potential determined using a surface force apparatus for solid hydrophobic surfaces immersed in dodecylammonium chloride solutions.