A sorption calorimetric method proposed by Wadso and Markova allows determination of water activity. Calculation of water activity is based on Fick's first law. The correct form of the Fick's law takes into account not only vapour diffusion itself but also bulk flow of vapour. A new formula, which uses the correct form of the diffusion law for direct calculation of water activity, is proposed. (C) 2003 Elsevier B.V. All rights reserved.