Except for areas in California, Libby, Montana is the only designated EPA nonattainment area for fine particulate matter (PM2.5) in the mid and western states. During the winter of 2003/2004, PM2.5 speciated data (mass, elements, ions, organic/elemental carbon) were collected every six days from November 11, 2003 through February 27, 2004. Using a Chemical Mass Balance computer model (Version 8.0), these data were used to apportion the sources of PM2.5 in the Libby valley. In support of the source apportionment program, a comprehensive evaluation of the particulate matter associated organic compounds (including polar organics, phenolics, polycyclic aromatic hydrocarbons, and 14C) present in the airshed was also conducted. CMB modeling results revealed that emissions from residential wood combustion was the major source of PM2.5 throughout the winter months in Libby, contributing an average of 82% of the measured PM2.5. Levoglucosan, a well-known chemical marker for wood smoke, had the highest measured concentrations of any of the 95 polar organic compounds quantified from the fine fraction, accounting for over 15.5% of the measured organic carbon fraction. Other semi-volatile organic compounds with high measured concentrations during the program were four phenolic compounds commonly found in wood smoke, including phenol, 2-methylphenol (o-cresol), 4-methylphenol (p-cresol), and 2,4-dimethylphenol. Results from 14C analysis indicate that as much as 82% of the measured 14C results from a wood smoke source. These indicators support modeling results that residential wood combustion was the major source of PM2.5 in Libby, Montana throughout the winter months.