Detailed gaseous and particle emissions along with thermal efficiency measurements were made on three mid-sized high-efficiency wood boilers with thermal output capacities of 150 kW (514 000 Btu h(-1)) (n = 2) and 500 kW (1.7 mm Btu h(-1)) (n = 1). Wood chips and commercial wood pellets were used as fuel. Continuous emissions of CO, NOx, SO2, fine particle mass (PM2.5), and ultrafine particle number distributions were determined using a dilution tunnel sampling system. PM2.5 and semivolatile organic compound characterization was performed. Low concentrations of CO, organic carbon (OC), and elemental carbon (EC) during steady-state boiler operation indicated good combustion conditions. Fine particle mass from wood pellets was predominantly K+ and SO42-, with < 8% OC and < 2% EC. Inorganic emissions (PM2.5, NOx, and SO2) were found to depend upon fuel quality, which indicates the need for wood pellet fuel standards in the U.S. Cd, Pb, Ti, Rb, and Zn were found to be enriched in PM2.5, which is of concern for human health. Levoglucosan was a predominant organic compound found for all fuels, ranging from 38 to 82 mu g/MJ. Total particle and semivolatile polycyclic aromatic hydrocarbon (PAH) emissions were relatively low (19.4-92.8 mu g/MJ). The thermal efficiencies of the wood pellet boilers determined using the provisional American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) standard 155p ranged between 70 and 86% for the 150 kW boiler tested and between 75 and 91% for the 500 kW boiler. The use of ASHRAE 155p allowed for the determination of the linear relationship between the energy input and the energy output over the range of energy outputs rather than only determining the efficiency at minimum and full load as is the current standard practice. Boiler capacity had no significant effect on efficiency; however, the operating conditions, such as fuel feed rate, outlet water temperature, and building demand, did affect the results.