Studies of the structure changes in heated (20–200°C) and γ –irradiated (109- 5×109R, E = 1.3 Mev, Tirrad. = 40°C) crystals CsNO3, RbNO3and NaNO2have been carried out by means of X-ray diffraction methods. The investigation of temperature effect on the crystal structure has shown that the phase transformation in CsNO3and NaNO2are of continuous type while that in RbNO3is discontinuous. It has been found that crystal structures of CsNO3and NaNO2change under irradiation in the same way, as they change under heating, No changes of RbNO3crystal structure caused by irradiation have been found. Experimental results agree with an assumption that radiation-induced structure changes of high-temperature type can be observed only in the compounds, the phase transition of which is of continuous type.