The selection of a suitable sludge which will be the inoculum of the reactor is the ®rst step in the start-up procedure of an anaerobic reactor. The sludge selected (inoculum) for this study was obtained from a UASB (Up-¯ow Anaerobic Sludge Bed) reactor used for the treatment of alcohol industry wastewaters. The industrial sludge was used in this work as the inoculum of two laboratoryscale UASB anaerobic reactors, which were fed with different substrata. The feed for the ®rst reactor was acidi®ed substratum (volatile fatty acid) and the second reactor was fed with non-acidi®ed substratum (glucose). The purpose of this work was to study the effect of both substrata in regular operation and to analyse the ¯otation problem which arises when the reactor is fed with a non-acidi®ed substratum. Finally, the characteristics of the adapted sludge were studied and they were compared with the initial inolucum (sludge). The following observations were noted in relation to the evolution of the sludge during operation: density, TSS content and C, H and N contents remained similar, but the particle size and VSS/TSS increased. The settling volume index decreased and methanogenic and acidogenic activities increased during operation in both reactors. The reactor fed with volatile fatty acids was able to recover from unstable periods faster than the reactor fed with a non-acidi®ed substratum. The use of an acidi®ed substratum avoided ¯otation, stabilised the system and reduced the solid content in the ef¯uent. Moreover, it worked properly with partially acidi®ed in¯uents meaning a reduction in the chemical reactive expense required to control acid pH-value throughout operation.