Abstract Discharge mechanisms and characteristic parameters of halogen-quenched GM counters are extensively dependent on the composition of the fill gas. To determine the partial pressures of the different gas species present in the counter's filling mixture, a sampling device with a molecular leak and a QMS was used. Molecular flow was tested and the system was calibrated using N 2Ar and N 2-Ne mixtures. Fragmentation factors and relative sensitivities were determined for argon and neon. In the presence of chlorine, HCI was detected systematically. Fragmentation factors for Cl 2 and HCl could be determined but not the system's relative sensitivity for chlorine. The origin of HCI is discussed on the basis of the data collected.