The extension of the lean stability limits of gasoline-air mixtures using a microwave-assisted spark plug has been investigated. Experiments are conducted on a 1200 RPM single-cylinder Waukesha Cooperative Fuel Research (CFR) engine at two compression ratios: 7:1 and 9:1; and four different levels of microwave energy input per cycle (prior to accounting for transmission losses): 0 mJ (spark only), 130 mJ, 900 mJ, and 1640 mJ. For various microwave energy inputs, the effects upon stability limits are explored by gradually moving from stoichiometric conditions to increasingly lean mixtures. The coefficient of variation (COVIMEP) of the indicated mean effective pressure (IMEP) is used as an indication of the stability limits.Specific characteristics of microwave-assisted ignition are identified. Microwave enhancement extends stability limits into increasingly lean regions, but slow and partial burning at the leanest mixtures curb efficiency gains. Microwave assistance decreases occurrence of misfire and partial-burn by increasing early heat release in very lean mixtures, but engine operation is unaffected at closer-to-stoichiometric conditions. Increasing microwave energy input can improve combustion characteristics as compared to low-energy microwave enhancement, but a point is observed beyond which increased microwave energy input provides limited benefits. Microwave-enhanced ignition technology could provide an effective means of enabling increasingly lean combustion in spark-ignited engines, allowing reduced throttling losses and improved efficiency. Unfortunately, in the slow-burning CFR engine setup, reduction in emissions of oxides of nitrogen (NOx) through lean burn is not sufficient for compliance with current regulations, making exhaust aftertreatment a necessity.