An ultrasonic device was calibrated to measure in situ and in real time the polyol ester oil (POE) concentration of the refrigerant liquid R 410A. The first part of this paper is devoted to the properties of the mixture, to the effects of the presence of oil on the speed of sound in the liquid phase and to the calibration and validation procedures carried out with a saturated liquid refrigerant. In order to have a number of calibration points that is not too large, it is necessary to maintain the mixture as close as possible to saturation conditions, which constrains the choice of the location of the sensor on the installation investigated. In the second part, the first results obtained on this installation are presented. It appears that the speed of sound in the POE / R 410A mixture is a strong function of the temperature and oil concentration, as was expected, but it also significantly depends on the pressure. Consequently, if the use of a sensor in a sub-cooled area is considered, additional calibration and validation procedures are necessary.This paper was presented at the ECOS'00 Conference in Enschede, July 5-7, 2000