A diffusional model of radiocaesium fixation in soils and bottom sedim
ents was developed. According to this model fixation is a result of ra
diocaesium cations diffusing into the surface layer of soil or sedimen
t particles. Good agreement between predicted and experimental time de
pendences of Cs-137 fixation rate in soils indicates that the proposed
model seems adequately to reflect the actual mechanism of radiocaesiu
m fixation and can be used for prediction of this radionuclide's long-
term behaviour in natural soil-water systems. Its major advantage is a
small number of parameters which have a clear physical meaning and ca
n, in principle, be determined in an independent experiment or assesse
d theoretically.