A comparative study was carried out on the anaerobic digestion of untreated and previously-fermented (with Penicillium decumbens) beet molasses. Four continuous stirred tank reactors were used for the study, two with freely suspended biomass, and the other with biomass supported on saponite. The reactors operated satisfactorily between hydraulic retention times (HRT) of 53.3-10.6 days and 15.4-3.1 days for untreated and previously-fermented molasses respectively. The anaerobic digestion processes of untreated and pretreated molasses were found to follow first-order kinetics for biomass loading rates in the range of 0-0.55 and 0-0.75 g chemical oxygen demand (GOD) g(-1) volatile suspended solids (VSS) day(-1) respectively. The experimental data [namely unitary conversion or efficiency (X), HRT, biomass concentration (M) and incoming substrate concentration (S-0)] conformed to an equation of the form : X/HRT = KM(1 - X)-(KMSR/S-0), from which the kinetic constant, K, was calculated. The kinetic constants were influenced by the pretreatment carried out and were 1.7 and 2.5 times higher for pretreated molasses than for untreated molasses in the reactors with suspended and immobilized biomass respectively. This was significant at a 95% confidence level. The specific rate of substrate uptake for cell maintenance (m) decreased by a factor of approximately 2 for the previously fermented molasses in relation to the observed values for the untreated molasses. This may be attributable to the fact that higher phenolic compound concentrations inhibit and interfere with the activity of anaerobic bacteria.