Two forms of the hygroscopic behaviour of soybean protein isolates (SPI): liquid water imbibing capacity (WIC) and equilibrium vapour adsorption were tested in non-dialysed native (N) and denatured (D) samples, and in corresponding dialysed isolates (ND and DD). Water activity (a(w)) was measured by the optical condensation dew point method. The GAB model adequately predicted the SPI experimental sorption isotherms. The greater number of polar groups exposed during denaturation along with the salt-induced protein aggregation cause the stronger adsorption on D isolate compared with N. No differences between the isotherms up to very high aw were observed between ND and DD isolates. WIC of isolate D was greater than in N and this correlated with higher moisture adsorbed in equilibrium. Denatured non-dialysed isolates may be stored safely at higher moisture contents, and constitute a suitable ingredient for processing nutritionally-enriched formulated foods requiring high water retention. (C) 2002 Elsevier Science Ltd. All rights reserved.