The removal of undesirable volatiles from highly viscous polymer melts is a crucial and expensive task in polymer processing and manufacturing. This thermal separation process is applied several times over the life cycle of polymers. Residual monomers, solvents and other impurities can affect the quality of the final products. In addition, these often toxic substances are subject to strict environmental and health regulations. For the degassing of polymers usually twin-screw extruders are used. Good mixing and frequent surface renewal are important for an efficient devolatilization. At very low residual concentrations, mass transfer is diffusion-controlled and bubble-free. This mechanism is the topic of few publications with even less experimental data concluding that the measured mass transfer is considerable lower than predicted by theoretical models. In order to investigate the reasons for these observations, this contribution presents an experimental study of mass transfer during polymer devolatilization in a twin-screw extruder. All measurements are operated at ambient temperature with a highly viscous model substance system consisting of polydimethylsiloxane as polymer and 1,1,2-trichloro-1,2,2-trifluoroethane as volatile. A stripping gas flow of nitrogen is used to prevent foaming due to the supersaturation of the polymer. Gas flow rates are varied to estimate the influence of the gas sided mass transfer resistance, which in contrast to the literature is not negligible in all cases. A transparent barrel wall of the extruder allows to observe the fluid flow at all times. A systematic variation of rotational speed and degree of fill is performed. The development of mass transfer is evaluated with several sampling points in the devolatilization section. The concentration of volatiles in the melt is measured by thermogravimetric analysis. The results show an explanation for the deviations mentioned in the literature.