Continuous flow and batch experiments were carried out under mesophilic conditions (35 degrees C) to determine the effects of gradual increases in cation concentrations on specific methane producing activity (MPA) and microbial community structure in an upflow anaerobic sludge blanket (UASB) reactor fed with molasses wastewater for 300 days. The cation inhibition assay based on MPA as a function of potassium concentration was performed using the retained sludge. The 50% inhibitory concentration for potassium was 600 mM for H-2/CO2 substrate, 300 mM for acetate, and 280 mM for propionate, and inhibition levels were clearly proportional to potassium concentration. The tolerance of retained sludge to potassium inhibition was not clearly changed from low to high saline conditions (day 59, 111, and 201). The 16S rRNA gene sequencing analysis revealed that Methanobacterium and Methanosaeta were the dominant archaea, and the archaeal community was not obviously changed at the genus level during operation. (C) 2017 Elsevier B.V. All rights reserved.