Much of human exposure to particulate matter (PM) of ambient origin occurs indoors. While the oxidative potential (OP) of PM is an emerging indicator of the intrinsic toxicity of PM, little is known about the OP of indoor PM of ambient origin. Here we characterize the OP of indoor and outdoor size-resolved PM in an unoccupied apartment unit in Chicago, IL, free from indoor emission sources, to explore how the transport of ambient PM from outdoors affects OP indoors. Simultaneous measurements were conducted in both indoor and outdoor locations for at least 5 consecutive days during 11 separate sampling campaigns spanning all seasons. OP was assessed by the dithiothreitol (DTT) assay. Indoor PM_2.5 concentrations were consistently lower than outdoor PM_2.5 concentrations, with an average (±standard deviation, SD) indoor/outdoor (I/O) PM_2.5 mass concentration ratio of 0.36 ± 0.10. The volume-normalized (or extrinsic) OP of the particles was much higher for the outdoor for PM_2.5 samples across all size-ranges, with an average (±SD) indoor-to-outdoor ratio (I/O) of 0.54 ± 0.57. However, the mass-normalized (or intrinsic) OP of indoor PM_2.5 samples was greater than concurrent outdoor PM_2.5 samples during 7 of the sampling periods and had an overall average ± SD I/O of 1.38 ± 0.97. The extent of enhancement in intrinsic OP of ambient-infiltrated indoor PM_2.5 was positively correlated with differences in indoor and outdoor temperature and relative humidity (RH). We hypothesize that changes in the intrinsic OP of ambient PM as it infiltrates indoors may be influenced by transformation processes such as size-resolved penetration, indoor deposition, evaporation of volatile chemical components with lower OP, and/or temperature/RH-dependent partitioning of redox-active substances to indoor-infiltrated PM. However, further investigations, including chemical analyses of the size-segregated indoor and outdoor PM samples and the effect of environmental factors on this chemical composition, will be needed to confirm these hypotheses.