UNLABELLED: During 2005-2007, a woodstove changeout program was conducted in a Rocky Mountain valley community in an effort to reduce ambient levels of PM(2.5). In addition to changes in ambient PM(2.5), an opportunity was provided to evaluate the changes in indoor air quality when old stoves were replaced with US Environmental Protection Agency (EPA)-certified woodstoves. PM(2.5) samples were measured in 16 homes prior to and following the changeout. For each sampling event, PM(2.5) mass was continuously measured throughout the 24-h sampling periods, and organic/elemental carbon (OC/EC) and associated chemical markers of woodsmoke were measured from quartz filters. Results showed that average PM(2.5) concentrations and maximum PM(2.5) concentrations were reduced by 71% and 76%, respectively (as measured by TSI DustTraks). Levoglucosan was reduced by 45% following the introduction of the new woodstove. However, the concentrations of resin acids, natural chemicals found in the bark of wood, were increased following the introduction of the new woodstove. There were no discernible trends in methoxphenol levels, likely due to the semi-volatile nature of the species that were measured. Although there is some uncertainty in this study regarding the amount of ambient PM infiltration to the indoor environment, these findings demonstrated a large impact on indoor air quality following this intervention. PRACTICAL IMPLICATIONS: Emissions from residential woodstoves are an important air quality issue (both indoors and ambient) in many regions throughout the US and the world. More specifically, woodstoves have been identified as a major source of PM(2.5) in valley locations throughout the Northern Rocky Mountains, where biomass combustion is the predominant source of home heating. In this study, we present results that demonstrate the dramatic reduction in PM(2.5) concentrations (as measured by TSI, Inc. DustTrak PM(2.5) air samplers) inside homes following the replacement of old, polluting woodstove with new EPA-certified woodstoves.