Corn and wheat bran flakes were equilibrated to different water activities and subjected to compression in bulk. Relationship between both force and work of compression and water activity was analyzed using Peleg's and Fermi's equations. Scanning electron microphotographs were used to analyze microscopic structure of flakes. In both analyzed flakes an antiplasticizing effect of water was observed. Beyond water activity 0.6-0.7 plasticizing effect of water was noticed. In the antiplasticizing range of water activities response of flakes to compressing force was related to microstructure of the material. Relationship between both the force and work of compression and water activity in corn flakes was curvilinear and could not be approximated by the Fermi's equation. Corn flakes showed rather homogenous porous structure with continuous air cell wall matrix. Influence of water activity on both the force and work of compression in wheat bran flakes was well described by the Fermi's equation. The flakes showed microstructure with many discontinuities and resembling agglomerate. The antiplasticizing effect of water was less evident than that in corn flakes and the compression either force or work changed little with water activity until a critical value was reached. (c) 2005 Elsevier Ltd. All rights reserved.