The agglomeration phenomenon of amorphous particulate material is a major problem in the food industry. Currently, the glass transition temperature (T-g) is used as a fundamental parameter to describe and control agglomeration. Models are available that describe the kinetics of the agglomeration process as a function of the distance of the material from T-g (i.e. T - T-g). In practice these models are often not applied because they assume that the powder material is instantly in equilibrium with the humidity conditions of the environment and that solid mobility only occurs at T > T-g. Insights in the kinetics and mechanisms of water transport in powder material can help to better understand and control powder agglomeration. For this purpose, gravimetric step-change water sorption experiments were performed on maltodextrins as a function of the water activity a(w). The maltodextrins vary in dextrose equivalents (DE), particle size and morphology. The experimental results were compared with a Fickian diffusion model in order to understand the dependency of the transport mechanism on water concentration gradient and material relaxation. The water transport kinetics in the maltodextrins with low DE (i.e. 6) were well described by Fickian diffusion for low aw, independently of particle size and morphology, until relaxation phenomena started to occur at an aw corresponding to T - T-g = -20 degrees C. The importance of the matrix relaxation phenomena on the water transport mechanism increased with increasing DE (i.e. 29 and 32), not showing any relationship with the T. The results of this study indicate that the water migration mechanism is controlled by relaxation phenomena when the amorphous material is still far from the glass-rubber transition. The T - T-g at which the relaxation phenomena occur depends on the material. On the contrary, the T - T-g parameter could well describe the onset of agglomeration, independently from the material properties. Therefore, it can be concluded that within the conditions of this study, matrix relaxation occurring far below T-g did not affect the onset of agglomeration. (C) 2011 Elsevier Ltd. All rights reserved.