The phase transitions in Rb-doped KNO3 samples between the alpha, beta and gamma phases were followed by simultaneous acoustic emission (AE)-DTA measurement and high temperature X-ray diffraction. When the samples were cooled from 400 degrees C (above the melting point of 340 degrees C), the beta to gamma and gamma to alpha phase transitions shifted to a lower temperature and higher temperature, respectively, with increasing amounts of Rb dopant. When 0.5 mol%-doped samples were cooled from 300 degrees C, the ferroelectric gamma phase began to transform to the alpha phase at about 60 degrees C, and survived even at room temperature. The transition behaviour of the pure and 0.5 mol%-doped samples monitored by the AE technique was confirmed by high temperature X-ray diffraction.