The placing of stainless-steel or ceramic rods (or inserts) in the hot flame gases downstream of a burner is known to be a cheap and effective way of reducing NOx emissions from domestic gas burners. However, the mechanism of NOx reduction has not been fully identified. This paper describes an experimental study of NOx emissions from a domestic methane burner as a function of flow rate, air:gas ratio, rod diameter, rod material and rod position. In a fuel-rich flame, reductions of 40% in NOx emissions were obtained with a stainless-steel rod placed just downstream of the raction zone; an alumina ceramic rod gave reductions of only about 15%. in a fuel-lean (fully premixed) flame, however, no reduction was seen for either of these materials. Many of the observations can be explained by a ＇thermal＇ mechanism, whereby the insert removes heat from the flame, hence reducing the flame temperature and slowing the rate of formation of NOx. Further investigation is required into fuel-lean flames and the variation of NOx emissions with flow rate through the burner.