The 'dose-rate effect' has been studied in potassium dihydrogen phosphate (KDP) single crystals as a realisation of the peculiar mechanism of radiation defect formation in soft-mode crystals. The temperature dependence of the effect, calculated analytically on the basis of model concepts, is in agreement with that obtained experimentally. The possibility of the occurrence of a similar dose-rate effect during irradiation of highly correlated systems including chalcogenide glasses, charge-ordered systems and Mott's systems has been discussed.