This paper evaluates the formation of NO in a non-pressurized, water-cooled combustion chamber. Thermal and prompt NO were roughly estimated from local concentrations of NO in the fumes, and were studied at different burner operating points (combustion pressure, equivalence ratio, fuel spray pattern, and swirl angle). Afterwards, thermal and prompt NO for six different methyl esters and petrodiesel were measured, compared and what mostly influences and differentiates thermal and prompt NO between fuels was identified. Results reveal that the level of thermal NO for fuels is almost the same order of magnitude, while prompt NO varies markedly among the fuels. The relative level of thermal and prompt NO is much different depending on the operating points of the burner varying from 16/84 to up to 73/27. (C) 2016 Elsevier Ltd. All rights reserved.