The soaking process of a grain amaranth variety (Amaranthus cruentus) was investigated in water between 30 and 90 degreesC. A mathematical model describing simultaneous unsteady water diffusion and first-order irreversible water starch reaction is proposed to simulate soaking process of grains. The observed water absorption curves are used to estimate the effective diffusivities (D-ef) and reaction rate (k). The values of D-ef and k were between 10(-12) and 10(-11) M(2)s(-1) and 10(-5) and 10(-3) s(-1), respectively. The dependence on temperature of D-ef and k was evaluated through an Arrhenius type equation with two distinct activation energies for diffusion (E-D) and reaction (E-k) below and above 64 degreesC. The calculated values were E-D=31.0 kJmol(-1) and E-k=24.5 kJmol(-1) below 64 degreesC and E-D=53.6 kJmol(-1) and E-k= 136.5 kJmol(-1) above 64 degreesC. These results suggest that at least, for the soaking temperatures above 64 degreesC the absorption process is controlled by water-starch reactivity. (C) 2004 Published by Elsevier Ltd.