Industrial and fundamental aspects of oil combustion in highly preheated air were studied in experiments carried out at 0.58 MW fuel input with combustion air preheated to 1300 degreesC. Both a light and a heavy fuel oil were used. The oil injectors were positioned away from the combustion air stream. Detailed in-furnace measurements of temperature, chemistry (O-2, CO, CO2, H-2, Unburned Hydrocarbons, NOx, soot and particulates) and heat transfer were performed. A high uniformity of the temperature field and radiative fluxes was observed. Combustion of the light oil takes place without a visible presence of flame. Although the furnace was operated with an overall excess air of 10%, the combustion of evaporated oils occurred in strongly sub-stoichiometric conditions due to the entrainment of combustion products into the fuel jets. Despite the high air preheat, the NO furnace emissions were low (95 ppm for the LFO and 215 ppm for the HFO). This is attributed to suppression of peak temperatures and to the presence of an efficient NO-reburning mechanism. Excessive emissions of soot and particulates were observed with HFO firing. It was concluded that specific atomizers should be developed for such combustion process.