Thin polymer films are widely studied because of their countless technological and industrial applications such as protective coatings, lubricants, adhesive layers, insulating layers in microelectronics… When the thickness of these films decreases and approaches the characteristic length of individual polymer chains, confinement effects and interactions due to the presence of two interfaces are likely to profoundly modify various physicochemical properties. During the last two decades, numerous studies have shown that the glass transition temperature (Tg), the coefficient of thermal expansion (CTE), the physical aging or the wetting are deeply modified with respect to their bulk counterpart. Among these key properties, the possible variations of the density remain highly controversial and debated because most of the recent evaluations have been done indirectly by methods depending on a specific model (ellipsometry, XRR…). Recently, two teams from the Paul Pascal Research Center (CNRS/University of Bordeaux) and the Dupuy de Lôme Research Institute (CNRS/University of Southern Brittany) have solved this enigma by developing a new experimental technique of dissolution of plastic films monitored by a quartz crystal microbalance (QCM). This approach allowed to directly measure subtle changes in density as a function of film thickness for two very common polymers: Polystyrene (PS) and Polymethylmethacrylate (PMMA). This groundbreaking work has clearly shown a significant variation in density when the thickness of the films is reduced below a hundred nanometers. An increase for PS and a decrease for PMMA were observed, in agreement with recent work on the variation of the optical properties of these films.