The oxidative potential of size-segregated particulate matter (PM) samples collected inside and outside of an unoccupied apartment unit was measured based on a dithiothreitol (DTT) assay. The indoor PM mass was concentrated in the smaller size range (<0.5 µm), while outdoor PM was mostly in the larger sizes (1.5-7.0 µm). An overall higher (2-5 times) intrinsic DTT activity was observed for the indoor PM than outdoor for all size ranges, but the indoor/outdoor (I/O) ratio peaked at around 1.0 µm. In contrast, the volume-normalized DTT activity was higher for indoor samples than outdoor in smaller size range (<1 µm), but was similar for the coarse particles (2.5-10 µm). The results, consistent with penetration and deposition theory, indicate that the infiltration and persistence of outdoor PM concentrates mass in the ~0.1 to ~1 µm size range in the indoor environment. This may contribute to the substantial elevation of the intrinsic oxidative potential of indoor ambient PM.