Thermal-hydraulic test facilities are widely used to investigate various containment phenomena as, for example, mixing and stratification of gases or steam condensation in the presence of noncondensibles. Experiments are also required for validation of codes possessing capabilities for modelling such three-dimensional effects. The need for advanced instrumentation allowing to measure gas concentration in such conditions (typically: 100-150°C; 1-10 bar) and to get sufficiently refined information about spatial distribution of the different gas species has become apparent. Quadrupole Mass Spectrometers (QMS) have often been selected for this purpose because of attractive metrological characteristics (sensibility, span of concentration range, response time, stability, etc.), relatively compact size and low cost. This paper deals with the implementation of gas analysis systems using some commercial QMS that have been recently added to the basic instrumentation in three thermal-hydraulic test facilities namely MISTRA (CEA, France), TOSQAN (IRSN, France) and PANDA (PSI, Switzerland). This work concerns measurement of steam-air(nitrogen, oxygen)-helium mixtures of any composition distributed in pressurized vessels. The developed systems provide concentration point measurements at typically a few tens of different locations in the facility vessels. This paper will first give an overview of the three different configurations and will evocate the main difficulties encountered to develop these systems. For example, these systems should be able to bring the sample from a pressurized vessel up to the analyser chamber where low vacuum conditions are established, and at the same time prevent any steam condensation to occur all along the sample line.