The goal of this project is to develop a physically-based, semi-empirical model that describes the concentration of indoor concentration of PM-2.5 (particle mass that is less than 2.5 microns in diameter) and its sulfate, nitrate, organic and black carbon constituents, derived from outdoor sources. We have established the methodology and experimental plan for building the model. Experimental measurements in residential style houses, in Richmond and Fresno, California, are being conducted to provide parameters for and evaluation of this model. The model will be used to improve estimates of human exposures to PM-2.5 of outdoor origin. The objectives of this study are to perform measurement and modeling tasks that produce a tested, semi-mechanistic description of chemical species-specific and residential PM-2.5 arising from the combination of outdoor PM and gas phase sources (HNO3 and NH3), and indoor gas phase (e.g. NH3) sources. We specifically address how indoor PM is affected by differences between indoor and outdoor temperature and relative humidity. In addition, we are interested in losses of particles within the building and as they migrate through the building shell. The resulting model will be general enough to predict probability distributions for species-specific indoor concentrations of PM-2.5 based on outdoor PM, and gas phase species concentrations, meteorological conditions, building construction characteristics, and HVAC operating conditions. Controlled intensive experiments were conducted at a suburban research house located in Clovis, California. The experiments utilized a large suite of instruments including conventional aerosol, meteorological and house characterization devices. In addition, two new instruments were developed providing high time resolution for the important particulate species of nitrate, sulfate, and carbon as well as important gaseous species including ammonia and nitric acid. Important initial observations include the result that, with rare exceptions, there is virtually no nitrate found inside the house. This nitrate appears to dissociate into ammonia and nitric acid with the nitric acid quickly depositing out. Initial model development has included work on characterizing penetration and deposition rates, the dynamic behavior of the indoor/outdoor ratio, and predicting infiltration rates. Results from the exploration of the indoor/outdoor ratio show that the traditional assumption of steady state conditions does not hold in general. Many values of the indoor/outdoor ratio exist for any single value of the infiltration rate. Successful prediction of the infiltration rate from measured driving variables is important for extending the results from the Clovis house to the larger housing stock.