The stability and rupture of thin aqueous films formed between a hydrogen bubble and a mercury droplet were studied in aqueous solutions of 5 x 10(-6) or 10(-3) M sodium dodecyl sulfate in the presence of 0.05 M Na2SO4. The mercury droplet was positively polarized at E = 0.2 V in the rational potential scale. The critical thickness of film rupture, or the equilibrium thickness of stable films, was measured using the microinterferometric method. Results were discussed in terms of DLVO and heterocoagulation theory by referring to the differential capacity previously reported.