<div class="htmlview paragraph">The Engine-Out Particulate Matter (EOPM) was collected from a spark ignition engine operating in steady state using a heated quartz fiber filter. The samples were weighted to obtain an EOPM_index and were analyzed using Scanning Electron Microscopy. The EOP M_index was not sensitive to the engine rpm and load. When the mixture is very rich (air equivalence ratio λ less than ∼ 0.7), the EOPM comprise mostly of soot particles from fuel combustion. In the lean to slightly rich region (0.8 &lt; λ &lt; 1.2), however, the EOPM are dominated by particles derived from the lubrication oil.</div>