The time-dependent nature of glass transition related cohesion was studied for amorphous sucrose, maltose, glucose, galactose and fructose powders. The powders were made by freeze drying and the low molecular weight sugars (glucose, galactose and fructose) were made as mixtures with lactose. The blow test was used to measure the cohesiveness of the different powders with respect to time while the powders were exposed to a constant temperature and relative humidity environment. It was found that the rate of change of cohesiveness of these powders was directly related to the amount that the glass transition temperature (T(g)) was exceeded by (T - T(g)). That is, cohesion was related to the combined effect of temperature and moisture and not the individual temperature, water activity or moisture content of the powder. This work confirms that the mechanism for sticking and caking of amorphous sugars is through the phase change of the amorphous sugar from a glass to a rubber at temperatures above the glass transition temperature. Furthermore, it confirms that the rate of cohesiveness development is proportional to the T - T(g) value, that is, the greater the temperature above the Tg, the quicker the powders will develop liquid bridges which may result in caking. (c) 2005 Elsevier Ltd. All rights reserved.