The knowledge of vapor-liquid equilibrium data of mixtures with performance chemicals, such as ionic liquids (ILs), is highly important for technical applications. New insights into the utilization of quartz crystal microbalances (QCMs) for reliable absorption measurements in ILs are provided. The validity of the Sau-erbrey equation for rigid films and bulk liquids is widely discussed in literature. This work shows how to ensure the validity of the Sauerbrey equation during absorption measurements in thin liquid films. Furthermore, the advantages of the employment of rough electrodes for absorption measurements within thin liquid films in comparison to polished ones are described in detail. Important recommendations in application of the QCM systems regarding gravimetric measurements are presented.