A new analytical technique which employs hydrated antimony pentoxide to selectively remove Na from Na–K–Cl mixed fluids was successfully tested. With this technique, the uncertainty in the K determination of inclusion fluids by neutron activation analyses can be reduced to less than ± 10% compared to greater than ± 20% uncertainty which is typical in the conventional techniques.Na/K ratios of fluid inclusions in 14 quartz samples from gold–quartz veins at the O'Brien mine. Quebec, Canada were determined using this technique. The Na/K atomic ratio of the samples ranges from 0.9 ± 0.1 to 24.3 ± 3.9. Optical examinations of the fluid inclusions indicate that the variation in the Na/K ratios is caused by the presence of varying proportions of four different types of fluid inclusions: type I (pregold mineralization fluids: CO2 rich), type II (gold mineralizing fluids showing evidence of boiling at temperatures of 380°–300 °C), type III (postgold mineralization fluids; vapor – liquid two phase inclusions), and type IV (postgold mineralization fluids; halite bearing), Type II fluid inclusions appear to have Na/K atomic ratios of approximately 1.0, while type IV inclusions show Na/K ratios of over 24.