Synthetic wastewater was treated with a submerged membrane bioreactor to investigate the organic removal performance, as well as the behaviour of soluble microbial products during long-term operation. Satisfactory chemical oxygen demand (COD), total organic carbon (TOC) and biological oxygen demand (BOD) removal efficiencies were achieved, averaging over 90, 94 and 95%, respectively. The accumulation of TOC in the supernatant of the bioreactor and its degradation after 5 months of operation was observed. Among this accumulation, macromolecules with a molecular weight (MW)\100000 accounted for 34%. A decrease in the supernatant TOC corresponded to a shift from large to small molecules. The organic substances with a MW\100000 decreased from 34 to 16%, whereas those with a MWB30000 increased from 33 up to 52%. The accumulation proved to be inhibitory towards the metabolic activity of the activated sludge, as well as contributing to the poor membrane permeability of the mixed liquor.