Abstract: An exhaust gas reactor is disclosed which utilizes microwave energy to create a plasma whereby chemical reactions are promoted. Such reactions serve to reduce pollutant emission. The processor has a microwave-resonant cavity (6) formed either in an exhaust passage of a combustion chamber or in the combustion chamber itself, the cavity receiving in either case exhaust gases released by combustion in the chamber. Subjection of the exhaust gases to microwaves causes them to form a plasma in which chemical reactions take place.