This work reports on the physical and chemical characteristics of the ashes of biomass residues burned in air as well as in simulated dry oxy-combustion conditions. Three pulverized biomass residues (olive residue, corn residue, and torrefied pine sawdust) were burned in a laboratory-scale laminar-flow drop tube furnace heated to 1400 K. Olive residue resulted in by far the largest particulate yields both submicrometer (PM1) and supermicrometer (PM1-18)-whereas torrefied pine sawdust resulted in the lowest. The collected particulate yields of these two biomasses were analogous to their ash contents. The collected particulate yields of corn residue, however, were lower than expected in view of its ash content. To investigate the effects of the oxygen mole fraction and of the background gas, the O-2 mole fraction was varied from 20% to 60% in either N-2 or CO2. Submicrometer particulate matter (PM1) emission yields of all three fuels were lower in O-2/CO2 than in O-2/N-2 environments; they typically, but not always, increased with increasing O-2 mole fraction in either background gas. The background gas had little effect on the chemical composition of the PM1 particles. High amounts of alkalis (potassium, calcium, and sodium) as well as of. chlorine were observed in PM1. In addition, phosphorus and sulfur also existed in high amounts in PM1 from combustion of corn residue. Supermicrometer particles (PM1-18) yields exhibited no clear trend when the background gas was changed or when the oxygen mole fraction was increased. The composition of these particles reflected the bulk ash composition of the parent fuels.