Moisture sorption isotherms at +4 °C and +22.5 °C were obtained for β-casein after isolation and after 9 months of storage at -29 °C and +22.5 °C. Glass transition state diagrams (Tg vs. moisture) were determined for β-casein after storage. The results showed that effects of storage temperature on moisture sorption isotherms were varied; however, at any aw differences in moisture content were small (<0.03g H2O/g solids at high aw). β-Casein stored at -29°C had lower mo and Tg values than that of β-casein stored at +22.5°C. The glass transition temperatures for β-casein were above room temperature, even at aw = 0.76. Onset of stickiness occurred above aw = 0.76.