Ultrasonic pulse-echo systems are widely used to estimate properties of liquids and gases. A common principle is to use a buffer material (buffer-rod) fixed to the ultrasound transducer. Assuming the acoustic properties of the buffer-rod are known, it is then possible to calculate the acoustic impedance of the unknown material. A problem occurs if the temperature of the buffer-rod changes during the measurements, since the properties of the buffer-rod, such as the acoustic attenuation depends on temperature. If, however, the temperature is recognized, it is possible to compensate for this. In this paper we present a method based on speed of sound changes in the buffer-rod to estimate the temperature. With the resulting model we are able to estimate temperatures in PMMA for the interval 5°C to 60°C with a 0.1 °C accuracy (at a 95% confidence level).