The NO and N2O formation mechanism is studied starting from a single fuel particle burning under well-defined conditions up to a pilot-scale circulating fluidized bed combustor (CFBC). The fuel, petroleum coke, was the same in all tests and care has been taken to obtain chemical similarity between the different units: a formation rate unit, a laboratory-scale and a pilot-scale CFBC. A detailed single particle NO/N2O formation model has been developed and incorporated in a CFBC NO/N2O emission model. To thoroughly test the modeled NO/N2O mechanism, the iodine addition method has been used in all units.