A system has been developed and tested for continuously monitoring a natural gas stream for the beta activity of both tritium and 85Kr. The system uses a proportional counter whose sensitive volume is limited to about 24% of the total volume by a set of four separate cathode frameworks. This feature avoids a buildup of background activity due to the daughter products of 222Rn, which tend to plate out in insensitive regions of the counter. Appropriate pressure, temperature, and flow controls are provided to operate the system either in the static or flowing mode. In the latter, the residence time of the counting gas is about ten minutes. An 55Fe source (5.9-keV 55Mn x rays) is used for calibrating system gain. Output pulses from diagonally opposed anodes are summed separately, amplified, and processed by pulse height analysis and coincidence circuitry to provide upper and lower energy logic pulse outputs. Optimum discriminator levels are chosen to exclude pulses due to tritium events from the upper energy channel but to include therein a substantial fraction of pulses due to 85Kr events. Calibration constants and pulse distribution factors are determined for operating conditions of pressure (2.75 bars, 40 psia), temperature (38°C), and gain in the presence of the sample gas by dilution with standard gases made with commercial grade CH4 having known specific activities of either tritium or 85Kr. The system threshold (two sigma) for detection of either tritium or 85Kr has been determined to be about 2 × 10−3 pCi/cm3 in the absence of 222Rn and a factor of 3 greater with typical concentrations of 222Rn. For comparison, the radioactivity concentration guides (in air) are 3 × 10−1 pCi/cm3 for 85Kr and 2 × 10−1 pCi/cm3 for tritium. Estimates are given for the threshold for detection of either activity, tritium or 85Kr, in the presence of the other, 10% of the 85Kr activity for tritium and a few percent of the tritium activity for 85Kr.