This paper describes the results of a series of experiments executed at the International Flame Research Foundation (IFRF) for a consortium of gas companies. The main objective of the experiments was to investigate novel NOx-reduction methods and their impact on heat-transfer in glass-melting furnaces fired by natural gas. A section of a full-scale glass-melting furnace was scaled down and simulated at a semi-industrial scale, with representative thermal boundary conditions. The experiments concentrated on optimising the mixing process between the fuel jet and the combustion air. A wide variety of firing modes were employed to investigate the effect of mixing on NOx and thermal efficiency. Results suggested a number of practical NOx-reduction techniques applicable to high-temperature furnaces. Over the whole range of firing modes investigated, NOx levels varied in the range of 200-2500 ppm (0% O-2) for identical excess-air levels and air-preheat temperatures. The fuel-efficiency levels varied between 49 and 55% of the fuel input, dependent on the firing mode. It has been demonstrated that by optimising the mixing of fuel and air, significant reduction of NOx is achievable while maintaining thermal efficiency and heat-transfer characteristics within acceptable limits.