The foam stabilizing ability of anionic, cationic and nonionic surfactants are studied with two methods, involving constant and variable capillary pressure in the foam liquid phase: the method of determining the foam lifetime at constant pressure and the Ross-Miles test. It is reported that the most important parameters for foam stability are the capillary pressure and the type of the foam films. It is proved that the foam lifetime τpdetermined at constant capillary pressure is a more precise and defined characteristics of foam stability.