An integrated process of metal chelate absorption coupled with two stage bio-reduction using immobilized cultures has been proposed to continuously removal of NOx, and the effects of SO2, NO and O-2 concentration, gas/liquid flow rate on NOx removal efficiency were investigated. Although nitrogen-containing components, such as Fe(II)EDTA-NO, NO2- and NO3- in the scrubbing solution, inhibited the bio-reduction of Fe(III)EDTA obviously, it was feasible to abate the inhibition effect by using the two stage bio-reduction system, and thus to improve NOx removal efficiency. The removal efficiency decreased slowly with the increase of SO2, O-2, NO concentration and gas flow rate, and increased with the increase of liquid flow rate. Continuously operating for 18 days, a high removal efficiency around 95% was reached by using the two-stage bio-reduction system with immobilized microorganisms, while the value decreased to 85% after 5 days of operation by using the suspended microorganisms, at a constant gas flow rate of 60 L/h containing 424-450 mg/m(3) NO, 2428-2532 mg/m(3) SO2 and 3% O-2. (C) 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.