This study mainly investigated the conceptual and experimental aspects related to the assessment of the particulate inert COD in tannery, domestic and synthetic wastewaters. The assumption of a constant f(EX) coefficient defining the residual fraction of endogenous biomass was not justified. Experimental proof was provided for the effect of residual microbial products generation, both soluble and particulate, on the assessment of inert COD fractions of different wastewaters. The results enabled the description of a unified procedure for the experimental assessment of inert COD fractions of wastewaters, as well as residual microbial products. (C) 1999 Elsevier Science Ltd. All rights reserved.