To characterize the impact of the October 2007 wildfires on the air quality of Los Angeles, integrated ambient particulate matter (PM) samples were collected near the University of Southern California between October 24 and November 14, 2007. Samples were analyzed for different chemical species (i.e., water-soluble organic carbon, water-soluble elements, and several organic compounds), and the redox activity of PM was evaluated using two different assays: the dithiothreitol (DTT) and macrophage reactive oxygen species (ROS) assays. Tracers of biomass burning such as potassium and levoglucosan were elevated by 2-fold during the fire period (October 24−28), compared to the postfire period (November 1−14). Water-soluble organic carbon (WSOC) concentrations were also higher during the fire event (170 and 78 μg/mg of PM, during fire and postfire, respectively). While the DTT activity (on a per PM mass basis) increased for samples collected during the fire event (0.024 nmol DTT/min × μg on October 24) compared to the postfire samples (0.005 nmol DTT/min × μg on November 14), the ROS activity appears to be unaffected by the wildfires, probably because these two assays are driven by different PM species. While the DTT assay reflected the redox potential of polar organic compounds, which are abundant in wood-smoke, the ROS assay was mainly influenced by transition metals (e.g., Fe, Cu, Cr, Zn, Ni, and V), emitted mostly by vehicular traffic and other combustion sources, but not by the wildfires.