This study characterized the particulate matter (PM) pollution at a forest site in the Khao Yai (KY) National Park of Thailand, 700 m above sea level and 120 km upwind to Bangkok during the dry season. Two dichotomous samplers simultaneously operated on each monitoring day to collect 2 samples (on 2 filter types: quartz and mix cellulose) of 24 h PM2.5 and 2 samples of 24 h PM10–2.5. More focus was on the dry and more polluted season with a longer sampling period (43 days: Jan–Feb 2005) and less on wet season (10 days, Jun 2005). Samples were analyzed for mass, water soluble ions, elements, EC and OC (Sunset analyzer). PM2.5 levels obtained at KY in the dry period (47 µg m–3) were comparable to those in several urban areas in Southeast Asia, but the wet season levels (7 µg m–3) were significantly lower. In the dry season, levels of EC associated with PM2.5 at KY were remarkably lower than but OC were comparable to those found in urban areas which resulted in considerably low EC/TC ratios (0.08). The major mass groups of PM2.5 in the dry season were organic matter of biomass smoke origin (OM-smoke), secondary inorganic aerosol, organic matter of other origins than smoke (OM-others) and crustal. Similar contributors to PM2.5 were also found in the wet season but with only a small contribution from biomass burning smoke. Significant contributions from distant sources to PM2.5 levels measured at KY were also confirmed by the HYSPLIT backward trajectory analysis. The coarse fraction (PM10–2.5) had major mass groups of OM-others, inorganic particles and crustal that were most likely related to local sources, and some amount of aged sea salt indicating a distant source origin.