Experiments were conducted to understand competing NOx formation and destruction reactions better in circulating fluidized bed combustion. Both longitudinal and lateral gradients were measured for NOx, O-2, CO and CH4 during combustion of solid fuels of differing reactivity in a CFBC pilot unit 152 mm square by 7.3 m tall. Fuels tested were anthracitic, bituminous, subbituminous and lignitic coals and petroleum coke. In all cases there were significant lateral and longitudinal gradients of gaseous species. Profiles of NOx were correlated to a large degree with profiles of oxygen and local char concentration, which in turn were coupled to the CFB hydrodynamics. High partial pressures of oxygen at the base of the unit and in the char-lean core favour NOx formation reactions, while low O-2 partial pressures in the solids-rich annulus and towards the top of the unit favour NOx reduction. When NOx formation and destruction are limited to the char, profiles are readily interpreted. The addition of volatile nitrogen makes the profiles considerably more complex. Limestone used for sulfur capture can augment or reduce overall NOx formation, depending on the percentage volatile matter of the fuel.