The use of isothermal TGA with Knudsen-like cells allows determination of the thermodynamic activity of water, a(w). The typical experiment implies a slow dehydration of the aqueous solution at constant temperature in dynamic vacuum conditions. The method is alternative to the classical isopiestic approach and offers the advantage of a continuous record on increasing the solute concentration. These data can be directly treated according to the classical thermodynamic relationships drawn from the Gibbs-Duhem expression to evaluate the activity and osmotic coefficient of the aqueous solutions of electrolytes and non-electrolytes, and, in the case of electrolytes, allow determination of solubility of the solute. Discrepancies with respect to the literature data are observed when the viscosity of the systems becomes too high, as in the case of sugars with a very large solubility. Such a mismatch may however be accounted for either slowing the dehydration rate with use of a narrower Knudsen orifice, or correcting the experimental a(w) with a calibration curve. The same approach can be applied to non-aqueous solutions. (c) 2013 Elsevier Ltd. All rights reserved.