Biochemical methane potential (BMP) tests have been carried out on sewage sludge from two wastewater treatment plants to assess the effect of additives (FeCl(3) and two cationic polyelectrolytes) used in sludge dewatering. BMP tests were also carried out on the concentrated solid phase from the enzymic hydrolysis pre-treatment (42 °C, 48 h). FeCl(3) had no significant effect on specific methane production, obtaining 242-246 LCH(4)/kgVSo. The effect of the combination of polyelectrolyte and FeCl(3) depended on the polyelectrolyte and the sludge, but generally led to an increase in specific methane production (25-40%). When enzymic hydrolysis was applied as a pre-treatment, specific methane production increased from 6.8% in the sludge containing FeCl(3) to 20% in the sludge without FeCl(3), although the increases were not statistically significant. In terms of LCH(4)/kgVS(rem), a general improvement was achieved both by means of additives and by enzymic hydrolysis. However, this improvement was only significant in the case of sludge which had undergone previous enzymic hydrolysis (62%) and in the untreated sludge containing a polyelectrolyte and FeCl(3) (24%). Cationic polyelectrolytes inhibited solid-liquid separation during enzymic hydrolysis and, although the presence of only FeCl(3) did not affect this separation, a significant decrease (32%) in LCH(4)/kgVSrem was observed.