This work studied the effect of supplying trace metals (7.5 mu M Fe and 0.5 mu M Co, Ni, Mn, Zn, Cu, B, Se, Mo and W) on sulfate reduction and acidification in thermophilic (55 degrees C) UASB reactors fed with sucrose (4 gCOD (I(reactor) d)(-1)) operated at a reactor mixed liquor pH controlled at 5. Trace metals were supplied to one UASB reactor and were omitted from the influent of a second UASB reactor. The influence of different trace metal concentrations was further assessed in batch tests performed with the sludge from the UASB reactor receiving no trace metals. The absence of trace metals in the influent did not affect the performance of the acidifying UASB reactor throughout the 305 day long reactor run, but supplying low concentrations of trace metals inhibited sulfate reduction. (C) 2010 Elsevier Ltd. All rights reserved.