Results are presented from a study on NOx formation in rotary kiln flames. The work was performed on a simulator of a cement kiln at the International Flame Research Foundation. Both mono- and multi-channel burners were studied firing high- and medium-volatile bituminous coals and a petroleum coke. The secondary air preheat temperature was 800 degrees C throughout the experiments. The results show that a primary feature related to NOx formation in rotary kiln flames is the ignition distance. NOx emissions increased substantially as the flame ignition distance increased. The results are interpreted in terms of the amount of secondary air entrained into the primary fuel jet before ignition. Increased amounts of entrained air result in a more oxygen-rich flame propagation pattern and consequently higher flame temperatures and enhanced thermal NOx formation rates. Although all the fuels contained fuel-bound nitrogen, the results confirm that thermal NOx was the dominant NOx source in these studies.