This work determined theological parameters able to describe the theological properties of the flocculant sludge presents in sulphidogenic anaerobic bioreactors, i.e. a MBR (membrane bioreactor) and a CSTR (continuous stirred tank reactor). Both sludges displayed a non-Newtonian theological behaviour with shear thinning properties, slightly thixotropic for a TSS content ranging from 0 to 23 gl(-1). Both sulphidogenic sludges displayed pseudoplastic properties. To predict the theological properties of other sludges, three theological models (Casson, Bingham and Herschel-Buckley) were selected and fitted to the experimental theological data obtained. The mathematic equations of these three models give seven rheological parameters. To select relevant theological parameters, exponential and second-order polynomial equations were tested to evaluate the relationship between the theological parameters and the sludge TSS content. These two last variables were statistically fitted with a second-order polynomial correlation, which allows to select rheological parameters able to describe the evolution of the sludges in both bioreactors. The Bingham model applied on the pseudo-Newtonian area of a rheogram gives the more suitable rheological parameters to describe the evolution of the theological properties of anaerobic sulphidogenic flocculant sludge. The selected theological parameters of the sulphidogenic sludges are affected by the bioreactor type and its operation time. This is likely linked to changes in microbial consortia involving a change in biomass morphology and a decrease in floc strength. (c) 2006 Elsevier Inc. All rights reserved.