A device for treating a gas stream, especially an exhaust-gas stream of an internal combustion engine, includes at least one radial chamber radially conducting the gas stream and extending substantially radially from a central region to an outer collecting chamber. First and second, preferably parallel, substantially disk-shaped walls delimit the chamber. Electrodes project from the first wall into the chamber. The first wall is formed of electrically insulating material having the electrodes fastened therein and electrically interconnected by electrical conductors in or on the insulating material. Two or more chambers may be disposed, axially in series, around the central region. Very effective treatment of exhaust gas with an electric field for ionization or generating a plasma can be provided over a relatively short structural length in a flow direction. Electric fields running transversely to the flow direction provide new possibilities for selecting various parameters of the treatment device.