Foam films from aqueous solutions of an ABA triblock copolymer of polyethylene oxide and polypropylene oxide (M   =   14,000) are studied. A narrow polymer concentration range located below the CMC is investigated at both low and high electrolyte (NaCI) concentrations. The dependence of the surface force on film thickness is monitored by two complementary techniques: the porous plate method and the dynamic method of Scheludko and Exerowa. When the film thickness decreases, the total surface force is initially negative and after a minimum gradually increases to positive values. A transition from electrostatic to polymeric stabilization is induced by increasing the NaCI concentration. Our data can be fitted reasonably well with a combination of the DLVO theory ( van der Waals attraction and double layer repulsion) and de Gennes' scaling theory of steric interaction between grafted polymer layers.