This paper investigated the fate of soluble residual organics during biological treatment by membrane bioreactor. Particle size distribution analysis was evaluated as the main instrument for assessing the functional relationships among COD, TOC, proteins, carbohydrates, the aromatic content and the effective filtration size of the membrane. The study was conducted on black and grey water components of the wastewater generated from a small housing complex. The pore size of the selected membrane was 400 nm but it performed as a cake filter entrapping fractions larger than 5-14 nm for COD, 4-8 nm for TOC and proteins, and around 14 nm for carbohydrates. Around 50% of soluble residual organics, which were retained and accumulated in the reactor with a retention time equal to the sludge age, were removed due to longer exposure and possible acclimation of the biomass. Proteins and carbohydrates constituted around 60% of the effluent soluble residual COD for black water and 25% for grey water, at much higher ratios compared with the influent levels. Particle size distribution (PSD) analysis in the reactor bulk liquid, compared with parallel measurements in the effluent, offered a new dimension for the accurate evaluation of the fate and biodegradation of soluble residual organics. (C) 2010 Elsevier B.V. All rights reserved.