High ammonia nitrogen concentrations inhibit methanogenic activity and induce digester upset or failure. Therefore, it is essential to monitor the ammonium concentration in anaerobic systems. In this work, a fixed-point titration method was established to determine the ammonium concentration. This method was demonstrated to be effective with standard ammonium solutions, synthetic and practical reactor effluents, respectively. In addition, the effects of phosphate and ionic strength on titration accuracy were weak, while the interference of volatile fatty acids was inconspicuous. Considering its simplicity, accuracy and cost-effectiveness, the proposed titration method shows great potential for monitoring the ammonium concentration in practical applications.