This paper presents the crystal structure and the phase transitions of K x Na 1-x NbO 3 (0.4. X-ray diffraction measurements were used to follow the change of the unit-cell parameters and the symmetry in the temperature range 100-800K. At room temperature all the compositions exhibited a monoclinic metric of the unit cell with a small monoclinic distortion (90.32° °). No major change of symmetry was evidenced in the investigated compositional range, which should be characteristic of the morphotropic phaseboundary region. With increasing temperature, the samples underwent first-order monoclinictetragonal and tetragonal-cubic transitions. Only the potassium-rich phases were rhombohedral at 100 K.