The alkytrimethylammonium bromide surfactant series, C(n)TAB, n = 10, 12, 14, 16, is used to perform a systematic study of the forces and stability of foam films produced from soluble cationic surfactants. Both surface tension and disjoining pressure isotherms are measured for each surfactant. This information is then combined with neutron reflectivity and dynamic surface tension results found in the literature to provide an understanding of how the surfactant chain length can effect the forces and stability in thin-liquid films. For stable films, Re find good quantitative agreement between the interaction forces measured in foam films and those reported from surface force studies on similar systems. We also find that the surfactant’s hydrocarbon chain length and packing can strongly influence film stability. For highly purified surfactants an abrupt increase in film stability is seen when extending the chain length from C(12)TAB to C(14)TAB. When an uncharged cosurfactant is present, this stability transition takes place between C(10)TAB and C(12)TAB. Both transitions correlate well with changes in the monolayer’s cohesive strength, and we outline how monolayer cohesion can play a stabilizing role by dampening both spatial and density fluctuations at the air-water interface.