High salinity is one of the most well known inhibitors and salt concentrations above 1% are reported as highly saline., Salt may be found in the main collectors of municipal sewer systems which carry the risk of seawater infiltration and where domestic wastewaters are treated with landfill leachates such cases in Istanbul. Discharging of leachates into sewerage would also result in additional ammonium loads in Municipal Treatment Plants (MTPs). In this study, synthetic wastewater was fed to a lab-scale Upflow Anaerobic Sludge Bed (UASB) reactor operated at a constant hydraulic retention time (HRT) of I day. The salt concentration was increased artificially whereas the ammonium concentration was fixed at 1000 mg/l throughout the study. Results indicated no significant inhibition up to 2% salinity + 1000 mg/I NH4 and TOC removals could reach up to 88%. Besides, effective total biogas productions having methane content of 84% could be achieved. It is obvious that anaerobic microorganisms (especially methanogens) could well adapt to high salinity ratios. No inhibition at high ammonium concentration might have been attributed to the fact that the pH in the system was not high enough for the formation of free ammonia. Investigation of the combined effect of high salinity and ammonium was carried on by increasing the salinity to 3% in the study. Results indicated that TOC removals decreased to around 56% and 3% salinity caused a sharp decrease both in organic material removal and total biogas production. Thus the critical salinity level was determined as 3% for the methanogens in the mesophilic anaerobic digesters.