Abstract Tritium activity in water samples has been measured by means of a proportional counter. The energy interval was set between 1.1 keV and 9.7 keV to obtain optimal efficiency (the best figure of merit). The background count rate in the energy interval above 20 keV was measured simultaneously. A very good correlation between background count rates in both energy intervals has been observed. The counting gas quality has been checked by measuring the 55Fe spectral line (5.89 keV). A good correlation between the count rate above 20 keV and the spectral resolution has been obtained enabling gas quality control during the measurement. A constant monitoring of the count rate above 20 keV has revealed radon contamination of several groundwater samples. Repeated measurement after 15 days storage gave more accurate tritium activity.