The objective of this study was to evaluate the biodegradability related chemical oxygen demand (COD) fractionation of selected streams from a municipal wastewater treatment plant (MWWTP). The MWWTP under investigation consisted of an activated sludge process which was fed with domestic sewage and pretreated tannery effluents. Another aim was to use particle size distribution (PSD) analysis as a supportive tool for interpreting the effect of primary settling and biological treatment processes on the partitioning of COD fractions over different size categories. The results of conventional characterization and respirometric analyses revealed the strong character of the primary settling tank influent. The high level of soluble inert COD underlined the necessity of an additional treatment step, since the application of biological treatment alone was not adequate to meet the discharge standards. Primary settling removed 35% of the particulate COD, which corresponded to 70% of the initial total COD content, and exhibited only marginal COD removal (2%) in the soluble and colloidal size ranges. Biological treatment almost completely eliminated the particulate COD portion, translating into 60% contribution within 77% overall COD removal; where the COD decrease in the colloidal range amounted to a reduction efficiency of 12% on an overall basis. PSD-based COD fractionation proved to be a useful tool for evaluating the effect of primary settling and applied biological treatment process. PSD also complemented the results of respirometric analyses and provided supporting information for the interpretation of biodegradation characteristics by yielding the size distribution of major COD fractions.