The conceptual framework for the hydrolysis of slowly biodegradable substrate is reviewed by means of a model evaluation of the OUR profiles resulting from different rate expressions and the merit of each proposed mechanism is evaluated on the basis of experimental results. The sensitivity of the experimental evaluation technique, with specific emphasis on parameter identifiability, is investigated in assessing the values of the maximum specific hydrolysis rate, k(h) and the half-saturation coefficient for hydrolysis, K-x. The results of extensive studies for the experimental assessment of the hydrolysis rate coefficients for both domestic sewage and a number of industrial wastewaters are summarized. Furthermore, the question of organics with different hydrolysis rates is addressed by identifying readily and slowly hydrolyzable COD fractions. Dual hydrolysis approach is evaluated in terms of its implication on process kinetics and especially on the assessment of the endogenous decay mechanism. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.