The effect of very high-temperature exposure on the measurement properties of a fluorescence intensity ratio based optical fiber temperature sensor, using Nd3+-doped fiber as the sensing material, has been investigated. The results obtained indicate that exposing the sensor material to 750 °C for 100 h improves the accuracy of the fluorescence intensity ratio versus temperature characteristics from approximately 2.5 to approximately 1.3 °C. Measurements of the refractive index profiles of Nd3+-doped fiber samples prior to and after exposure to 750 °C indicate that such exposure results in small but measurable changes in the refractive index profile.