The strength of an adhesive bond depends on the thermodynamic work of adhesion, among other properties. In this paper, we report the direct measurement of the thermodynamic work of cohesion and adhesion between poly(ethylene terephthalate) (PET) and polyethylene (PE) films. The pull-off force between polymer surfaces was measured using the surface forces apparatus (SFA). Thermodynamic work of adhesion was determined from pull-off force measurements using the theory of contact mechanics developed by Johnson, Kendall, and Roberts (JKR theory). The values of the surface energies of PET and PE, and the interfacial energy between PET and PE were obtained from these measurements. The dependence of the measured values of the work of adhesion on the rate of separation, time in contact, and other variables that could reflect an irreversible contribution to the measured adhesion was found to be negligible. The critical surface tensions of PET and PE were determined from contact angle measurements. The critical surface tension of wetting depends on the characteristics of the probe liquids. The surface energy of PET determined by the direct force measurements is higher than the critical surface tension of wetting. These values are 61.2 mJ/m2 and about 43 mJ/m2, respectively. However, in the case of PE the surface energy determined using the SFA and the critical surface tension of wetting are about the same, 33 mJ/m2. The interfacial energy between PET and PE, obtained from direct measurements, is about 17.1 mJ/m2.