This paper describes experimental and modeling studies on the effect of metal-containing compounds on NOx reduction. Sodium and potassium carbonates, calcium acetate, and Ay ash were injected into a 300 kW combustor in one of the following configurations: with the main fuel, with the reburning fuel, or into the reburning zone. Natural gas was used as the main fuel and the reburning fuel. Tests showed that co-injection of Na-, K-, and Ca-containing compounds with the main fuel reduced NOx emissions with and without reburning. Co-injection of Na, K. and Ca-containing additives along with the reburning fuel and into the reburning zone downstream of the reburning fuel had a smaller effect than co-injection with the main fuel. Fly ash showed a minimal effect on NOx reduction. A potential benefit of using calcium-containing additives in reburning: is reduced SO2 emissions during coal combustion. About 50% SO2 reduction was achieved with injection of 1000 ppm of Ca(OH)(2) with the main fuel. Modeling was used to determine the mechanism of the effect of Na-containing additives on NOx reduction. The model combined a detailed description of the reburning chemistry with a simplified representation of mixing. Modeling suggested that reduction of NOx emissions in the presence of sodium-containing compounds was due to the inhibition of the combustion process by sodium. Because NOx formation via thermal and fuel-NO mechanisms strongly depends on the local combustion environment, NO, formation in the main combustion zone was inhibited because H. OH, and O concentrations in the combustion zone decreased in the presence of Na. The increase in NOx reduction in the reburning zone was due to slower oxidation of the reburning fuel in the presence of Na.