In order to examine the effect of easily degradable substrate such as acetate on toluene mineralization by denitrification, an upflow anaerobic sludge blanket (UASB) reactor in steady state was set up. The experimentation was carried out in two stages. Initially, the reactor was fed with a carbon loading rate of 250 mg acetate-C L-1 d(-1) as electron source. Nitrate loading rate (mg NO3- -N L-1 d(-1)) was adjusted to obtain a constant C/N ratio of 1.4. In the second stage, five toluene-C loading rates (TLR, mg toluene-C L-1 d(-1)), 25, 50, 75, 100 and 125, were assessed while total carbon loading rate and C/N were maintained constant at 250 mg C L-1 d(-1) and 1.4, respectively. In so doing, acetate-C loading rate (mg acetate-C L-1 d-1) was gradually substituted by toluene-C. When acetate-C was the only electron source a dissimilative denitrifying process resulted as indicated by bicarbonate yield Y-HCO3, mg HCO3- produced/mg carbon consumed) of 0.74 +/- 0.005 and denitrifying yield (Y-N2 mg N-2 produced/mg NO3- -N consumed of 0.89 +/- 0.042.3 The addition of different TLR did not affect the biological process as consumption carbon efficiency (CCE) values remained up to 95% +/- 3.5 and Y-HCO3 and Y-N2 values were higher than 0.71 +/- 0.03 and 0.88 +/- 0.01, respectively. Toluene mineralization by denitrification in continuous culture was successfully achieved. A simple UASB denitrifying reactor system has promising applications for complete conversion of nitrate, toluene and acetate into N-2 and CO2 with a minimal sludge production. (c) 2006 Elsevier Ltd. All rights reserved.