For a drastic reduction of heat transfer in glazing units low emissivity coatings and filling gases with low thermal conductivity, e.g. argon, krypton or xenon, are used. As the rim seal is not perfectly leak tight, part of the filling gas may diffuse out and air could diffuse into the spacing, resulting in a deterioration of the insulating performance. By measuring the sound velocity in the gas filling the relative amount of the noble gas can be determined. A theoretical expression for the sound velocity of an air-argon mixture is experimentally verified to 0.5%. In order to determine the krypton or xenon content to within about 10%, the sound velocity is determined by measuring changes of the travel time and the glazing spacing instead of absolute values. A change in spacing is achieved by pressing ultrasonic transducers onto the glazing unit with varying force. A sound frequency of typically 1 MHz is used for experiments carried out on a 50 × 50 cm2 argon filled double pane glazing with a spacing of 14 mm. The expected change of sound velocity with gas composition is analyzed with respect to the loss of thermal performance of the glazing. For xenon and krypton the effective U-value of the glazing can be determined from the ultrasound measurement with an uncertainty of better than 0.04 W m-2 K-1, whereas for argon the uncertainty with the present setup is acceptable only for higher air content of the glazing.