A study was made of the sorption properties of starch-protein extrudates produced at varied process parameters, with the use of water vapour adsorption/desorption. Isotherms of the processes were plotted, and the BET equation was used to determine the monolayer capacity and the specific surface area. The sorption isotherms for the raw materials and the extrudates were identical in shape, of type II acc. to IUPAC. Compared to the extrudates, the raw material always displayed higher absorption of water vapour. Irrespective of the kind of protein material, extrusion resulted in a lowering of water vapour monolayer capacity which varied from 0.066 g/g to 0.082 g/g for the raw materials. while for the extrudates it was somewhat lower and varied from 0.058 g/g to 0.062 g/g. Milder temperature conditions caused a slight increase in water vapour monolayer value. Correlations were determined between the specific surface area and the index of radial expansion; the specific surface area (S-BET) decreased with increasing expansion. Moreover, it was shown that the specific surface area of the extrudates increased with increase in their water absorption capacity. (c) 2007 Elsevier Ltd. All rights reserved.