Relevant experiments are essential to clearly understand the role of various molecular (chemical structure, surface energy, composition), experimental (contact time, contact pressure, temperature) or topological (sample roughness and thickness) parameters, on the tack properties of pressure sensitive adhesives (PSA). The "mechano-optical tack tester" (MOTT) is a novel device that we have developed to provide accurate measurements of both the contact area and the tack strength. The MOTT is designed to apply controlled contact pressures by mean of a quartz prism probe, for determined contact times, onto the surface of PSA samples. The probe is then pulled up at controlled rates while the tearing force (tack strength) and the contact area are plotted versus time. The tack energy is then calculated. Using the MOTT, the influence of various parameters (contact pressure, contact area, sample thickness,...) on the tack properties of PSA samples has been studied. The main result lies in the strong dependence of the tack energy on the sample thickness. This points out that the release energy is close to the interface rather than in the bulk of the PSA films, and is a function of the contact area.