In this study, the sorption of water in poly(methyl methacrylate) (PMMA) was measured at various water vapor activities (0–0.85) at 25, 35, and 45 °C using a quartz spring microbalance. Furthermore, the water sorption isotherms in PMMA were predicted using two nonequilibrium thermodynamic models: the nonequilibrium lattice fluid (NELF) model and nonequilibrium statistical associating fluid theory (NE-SAFT), where excellent agreement between the NE-SAFT prediction and experimental data was observed. In contrast, deviation between the NELF model prediction and water sorption isotherms in PMMA was observed above a water activity of ca. 0.50. In situ time-resolved Fourier transform infrared attenuated total reflectance spectroscopy confirmed the presence of self-associated water (i.e., water clusters) at elevated water activities, providing a rationale for deviation between the NELF model and experimental data, where unlike NE-SAFT, the NELF model does not account for these self-association interactions. The ...