[en] Fuel NOx and thermal NOx were measured in combustion gases from black liquors in two laboratory furnaces. Combustion at 950 C in air (8% O₂) produced NOx concentrations of 40-80ppm. Combustion at 950 C in synthetic air containing no nitrogen (21% 0₂ in Ar) produced the same result, demonstrating that all of the NOx produced during combustion at 950 C was fuel NOx. Formation of fuel NOx increased moderately with increasing temperature in the range of 800-1,000 C, but temperature sensitivity of fuel NOx was much less than that of thermal NOx. The results imply that the major source of NOx in recovery furnace emissions is the fuel NOx in recovery furnace formed by conversion of liquor-bound nitrogen during combustion. This is consistent with thermal NOx theory, which postulates that black-liquor combustion temperatures are too low to generate significant amounts of thermal NOx