The paper discusses retrofitting two 1.0 MW (3.5 million Btu/hr) firetube package boilers for fuel oil reburning application for NOx emission control. An overall NOx reduction of 46% from an uncontrolled emission of 125 ppm (dry, at 0% O2) was realized by diverting 20% of the total boiler load to a distillate-fuel-oil-fired second stage burner; a 51% NOx reduction from 265 ppm was achieved in a distillate/residual fuel oil mixture (0.14% nitrogen content) reburning application. Nitrogen-free fuel oil reburning was found to be slightly more effective at reducing NOx than natural gas reburning, although longer fuel-rich zone residence times were required to allow for evaporation and mixing of the fuel oil droplets. The key parameters investigated, that impact the reburning process, were: primary flame NOx, reburn zone stoichiometry, and reburn zone residence time. Reburning applied to firetube package boilers requires minimal facility modification.