This paper presents a microcalorimetric technique to simultaneously measure water activity and enthalpy of mixing (differential heat of sorption) as functions of composition at high water activities. The instrument consists of a sorption vessel in a double twin microcalorimeter. A sample at high water activity is placed in one chamber of the vessel and an unsaturated salt solution is injected into another chamber. A tube connects the chambers and diffusion will cause the water in the sample to be desorbed and condensed in the salt solution. As the rate of diffusion is high when the sample is at high water activity it is possible to make detailed studies of phenomena taking place at high water activity. The method has been tested on the system dimethyldodecylamine oxide (DDAO)-water. (C) 2003 Elsevier B.V. All rights reserved.