This study concentrates on the formation of fine particulate matter (PM) in a domestic pellet-fired boiler and includes a detailed characterization of the combustion process inside the combustion chamber of the boiler. Measurements of local mean gas temperatures, major gas species concentrations, and PM concentrations and size distributions are reported for a representative boiler operating condition. In addition, a number of selected PM samples were also morphologically and chemically characterized. The results revealed that (i) most particles present aerodynamic diameters below 1 mu m, regardless of the measurement location, and the mass size distributions peak in the size range of 50-130 nm; (ii) early in the flame, the PM size distributions present a bimodal size distribution, but this attribute vanishes as the final stages of the combustion process are approached; (iii) soot particles are rapidly formed early in the combustion process, but these carbon-rich particles also oxidize rapidly in the near burner region; (iv) the aerodynamic diameter corresponding to the maximum amount of particles increases slightly from the fuel bed until approximate to 4/5 of the combustion chamber height; (v) for a given measurement location and particle sizes below 1 pm, the PM chemical composition varies marginally among size ranges, indicating that the existing chemical compounds tend to form different size aggregates of similar composition; (vi) the element mass concentration varies according to the measurement location; early in the combustion process, the PM is dominated by carbon, with smaller amounts of O and Si, while in the final stages of the combustion process, the PM composition is dominated by inorganic material; and (vii) large fractions of the alkali metals are present as chlorides in the fine PM, and small fractions of the alkalis are present as sulfates.