We report frequency changes in AT-cut quartz crystals for glassy polymers subsequent to temperature and carbon dioxide pressure changes. Anomalous frequency shifts are observed for the crystal subsequent to such changes. Since, the Sauerbrey equation has been applied routinely for mass uptake measurements in glassy polymers, using the quartz crystal microbalance fitted with AT-cut quartz crystals, it is important that nongravimetric effects that impact the frequency change be well understood. In the present work, we provide a quantitative analysis of the breakdown of the Sauerbrey equation for viscoelastic materials by using the Johannsmann (Macromol Chem Phys 1999, 200, 501) treatment of the response of AT-cut crystals. Clearly, there exist significant errors in mass uptake measurements for materials deposited on AT-cut quartz crystals when precautions pertaining to film thickness and viscoelastic compliance are ignored. We show that while the early caution of EerNisse on stress effects in AT-cut quartz crystals can be important in mass uptake measurements, for polymers, the major source of error arises from viscoelastic effects in the coating. We conclude that mass uptake measurements for films of compliant materials and polymers above their glass transition cannot be performed with accuracy, using AT-cut quartz crystals if the results are not corrected for frequency shifts due to viscoelastic effects of the overlayer unless the films are extremely thin, that is, less than 100 nm. (c) 2006 Wiley Periodicals, Inc.