Nitrate contamination is becoming a widespread environmental problem, and autotrophic denitrification with Thiobacillus denitrificans is a promising process considering efficiency, cost and maintenance. The denitrification efficiencies of T denitrificans were compared in batch reactors between free cells and cells immobilized on polyvinyl alcohol (PVA) carriers made with thrice freezing/thawing and boric acid methods. The results indicated that the free cell reactor of T denitrificans added with 10% (v/v) of PVA carrier made by thrice freezing/thawing (PVA-TFT) exhibited faster in S2O32--S consumption, SO42- generation, and NO3--N denitrification, with corresponding values being 165 mg (S2O32--S)/L.d, 491 mg (SO42-)/Ld, and 44 mg (NO3--N)/Ld, which were increased by 50%, 61%, and 57% respectively compared to the control reactor with only free cells. Inhibition of denitrification by accumulated SO42- in PVA-TFT reactor appeared at the concentration of approximately 6000 mg (SO42-)/L, and 75% of NO3--N removal efficiency was achieved after 12 d operation under the condition of initial 700 mg/L NO3--N concentration. (C) 2008 Elsevier B.V. All rights reserved.