This paper reports on some recent work performed to establish the effectiveness of exhaust gas recycle as a knock suppressant in spark ignition engines, and to determine the effect of fuel and engine operating conditions on knock-limited engine performance and NOx emissions.Performance was evaluated with two different fuels: ASTM isooctane and ASTM 80 octane.With ASTM 80 octane fuel, operating near stoichiometric, the higher knock-limited compression ratio (klcr) resulted in a break mean effective pressure (bmep) which was constant for moderate recycle, falling only with high recycle rates. Brake specific fuel consumption (bsfc) decreased with recycle to a minimum, and then increased for higher recycle rates. For lean mixtures, however, bsfc increased steadily with recycle. Performance was similar with the isooctane. The performance recovery for stoichiometric and rich mixtures (low to moderate recycle) was attributed to the higher klcr and changes in the specific heat ratio. The observed degradation in performance recovery for lean mixtures and high recycle rates was due to losses associated with the lower burning rates.