Food-grade galacto-oligosaccharides (GOS) are commercially available as transparent syrups or dried powders. Food powders can be found in an amorphous metastable state which is very sensitive to changes in temperature and moisture content. In this work the impact of water content on thermal behavior and relative humidity on water sorption behavior of amorphous GOS powders were studied. Results from differential scanning calorimetry (DSC) and sorption isotherms suggest that GOS mixture studied, with high content of oligosaccharides, has low ability to crystallize. A dramatic decrease in the stability of GOS powders occurred above critical water content (12–14 g/100 g) and corresponding critical water activity (0.55–0.62). Above these conditions GOS powder lost its amorphous character, collapsed and shrank, as the powder became a transparent “solution-like” material. The knowledge about the physicochemical changes, acquired during the present study, should be used to a proper control of processing and storage conditions to achieve and maintain optimum powder quality with desired properties.