Ambient concentrations of fine particle extracted organic matter (EOM) measured at the Elm Grove Park and Fire Station sites in Boise have been apportioned to their two principal sources, woodsmoke and motor vehicle emissions. A multiple linear regression method using lead and potassium-based tracers for motor vehicles and woodsmoke, respectively, was employed in the source apportionment. On average woodsmoke was found to be the dominant contributor to EOM at both sites and during both day and night. In spite of the 90% reduction in the lead content of leaded gasoline in the U.S., which has been in effect since January 1986, lead still appears to be a viable tracer of motor vehicle emissions.