An investigation was made of the sodium lithium niobate Na_0.85Li_0.15NbO₃ ceramics using permittivity properties. Permittivity measurements were carried out by impedance spectroscopy in a frequency range of 5 to 1.3×10⁷ Hz and at temperatures ranging from 25 to 750 °C. The thermal stability of the dielectric behaviour was evaluated as a function of several thermal cycles. The permittivity response was found to depend on both the partial thermal cycle (heating or cooling cycle) and the number of cycles. Three phase transitions are proposed based on a maximum of permittivity response as a function of temperature. The dielectric behaviour and its relationship with the phase transition phenomenon are discussed.