The technique of tracer gas measurement has during recent years tended towards increasingly complicated measuring methods. The new measuring techniques are essential in order to procure more information about the circulation of air through buildings, or in order to perform more accurate measurements in large and complex buildings. The measuring method by means of "constant concentration of tracer-gas", which has been applied at Technological Institute for about 7 years, has proved to be a very accurate measuring method for both small and very large buildings. The method has the advantage of being able to continuously register the air change in a measuring area divided in numerous zones. The limitation of this method is that only information about the infiltration from outside into the measuring area is obtained, whereas no details are given about the air-flow between the individual zones of the measuring area. The limitation of the measuring method can be overcome by using 2 tracer-gases. The article describes the different philosophies on which measurements with "constant concentration of 2 tracer-gases" might be based, measurement result to be obtained and discusses whether there is any advantage of using more than 2 tracer-gases. In addition a specific measurement is described, where the method with "constant concentration of 2 tracer-gases" is used.