In this paper, a detailed account of the measurement and analysis of equilibrium isotherm data, obtained using a specialized device built for the purposes of large temperature and humidity ranges, is provided. The measurements were for the desorption isotherms of milk powder (skim milk powder and whole milk powder) for which data have not been available for the range that is relevant to spray drying conditions. The experimental approach allowed us to avoid the problem associated with amorphous lactose, which is known to undergo a glass transition if the equilibration process takes too long. The data points were adequately fitted using the GAB model, in terms of both the moisture content and the temperature dependence. These results will be widely used in spray drying simulations or spray drier designs for dairy powder production. (C) 2004 Elsevier Ltd. All rights reserved.