A vertical wetted-wall corona discharge reactor was used for removal of acetaldehyde in air. The reactor consists of a wire cathode sustained at the center of a cylindrical anode. Acetaldehyde laden air was fed either upward or downward through the wetted-wall reactor, in which water was circulated as a falling thin film on the inner wall of the anode. Ozone and short-lived species such as ions and radicals were generated in the reactor by gas corona. When some of these short-lived radicals drifted and reached the water film, reactive OH radical was produced in the water. Since gaseous acetaldehyde was readily absorbed into the water before the gas mixture entered the corona zone, decomposition of aqueous acetaldehyde by OH radical was considered as the main mechanism. 03 oxidation did not play a significant role in the present condition. It was found that there are a minimum current and a maximum inlet concentration of gaseous acetaldehyde for highly effective decomposition of aqueous acetaldehyde and TOC, resulting in steady state operation. It was calculated that one electron removed approximately 13 molecules of acetaldehyde. In comparison with the deposition type, the wetted-wall type exhibited clearly higher removal efficiency and lower byproduct formation. In addition, the effect of gas flow direction was discussed. (C) 2004 Elsevier B.V. All rights reserved.