Ultrasonic velocity measurement data has been established as a simple and convenient tool to determine different thermodynamic properties of liquids and solutions. Such measurements were carried out in situ to determine the solubility of NaN3 in liquid ammonia within a defined temperature range. Different mixtures of NaN3 in ammonia were prepared and the ultrasonic velocities of these mixtures were measured within this temperature range in order to compile calibration curves. Further, these calibration curves were used to calculate the solubility in a saturated solution.