This study describes the performance and stability characteristics of a parallel jet MILD (Moderate or Intense Low-oxygen Dilution) combustion burner system in a laboratory-scale furnace, in which the reactants and exhaust ports are all mounted on the same wall. Thermal field measurements are presented for cases with and without combustion air preheat, in addition to global temperature and emission measurements for a range of equivalence ratio, heat extraction, air preheat and fuel dilution levels. The present furnace/burner configuration proved to operate without the need for external air preheating, and achieved a high degree of temperature uniformity. Based on an analysis of the temperature distribution and emissions, PSR model predictions, and equilibrium calculations, the CO formation was found to be related to the mixing patterns and furnace temperature rather than reaction quenching by the heat exchanger. The critical equivalence ratio, or excess air level, which maintains low CO emissions is reported for different heat exchanger positions, and an optimum operating condition is identified. Results of CO and NOx emissions, together with visual observations and a simplified two-dimensional analysis of the furnace aerodynamics, demonstrate that fuel jet momentum controls the stability of this multiple jet system. A stability diagram showing the threshold for stable operation is reported, which is not explained by previous stability criteria. ; http://www.elsevier.com/wps/find/journaldescription.cws_home/505736/description#description ; G. G. Szegö, B. B. Dally and G. J. Nathan ; Copyright © 2008 The Combustion Institute