Stress relaxation in two varieties of potato tubers was studied with the aim of proposing a more suitable rate control model (RCM) for description of the relaxation process in vegetable tissue. This model was termed sigmoid model. It approximated the source f-function very well and gave about one order less model deviations from the experimental relaxation curves than the other usual simple models. It is shown that the best approximation accuracy can be obtained at higher pre-strains, but the highest inter-variety differences can be obtained at low pre-strains. The sigmoid model was rationalised by expressing the model parameters in ratio form. The behaviour of activation volume observed indicated that the sigmoid RCM covers some more complicated internal mechanisms responsible for relaxation process.