The first objective of this work was to evaluate the ability of the long-term BOD tests to provide the total biodegradable COD (BCOD) for domestic wastewaters. Results show that the method is repeatable (using two different volume samples, 97 and 164 mL) and that the fractions of BCOD determined were not statistically different from the one obtained by respirometry (at low S/X ratio). Respirometric tests were also repeatable. They were shown to be sensitive to the origin of the sludge. The results obtained are in the range of literature data. But they later indicated that long-term bioassays (.20 days) give higher biodegradable fractions than the other methods (BOD and respirometry). The second objective was to compare soluble fractions obtained with raw and pre-flocculated samples. Flocculated filtered COD is significantly lower than filtered COD, even if a pore size of 0.1 mm is used. The comparison to literature values shows that physicochemical RBCOD fractions are significantly higher than the ones obtained using bioassays. Comparison with the fractions used in calibrated models would help the choice of the more suitable method for modelling purposes.