Recent studies associate particulate air pollution with adverse health effects. The indoor exposure to particles of outdoor origin is not well-characterized, particularly for individual chemical species. In response to this, a field study in an unoccupied, single-story residence in Clovis, CA, was conducted. Real-time particle monitors were used both outdoors and indoors to quantity PM2.5 nitrate, sulfate, and carbon. The aggregate of the highly time-resolved sulfate data, as well as averages of these data, was fit using a time-averaged form of the infiltration equation, resulting in reasonable values for the penetration coefficient and deposition loss rate. In contrast, individual values of the indoor/outdoor ratio can vary significantly from that predicted by the model for time scales ranging from a few minutes to several hours. Measured indoor ammonium nitrate levels were typically significantly lower than expected solely on the basis of penetration and deposition losses. The additional reduction is due to the transformation of ammonium nitrate into ammonia and nitric acid gases indoors, which are subsequently lost by deposition and sorption to indoor surfaces. This result illustrates that exposure assessments based on total outdoor particle mass can obscure the actual causal relationships for indoor exposures to particles of outdoor origin.