Volatiles and char were prepared from the pyrolysis of a biosolid (with a phosphorus content of similar to 2.3 wt %) at 1000 degrees C and then combusted separately in air and oxyfuel (30% O-2 in CO2) in a drop-tube furnace at 1300 degrees C. The aim is to understand the effect of oxyfuel conditions on the emission of particulate matter with aerodynamic diameters of <10 mu m (PM10) from separated combustion in homo- and heterogeneous phases, respectively. For volatiles combustion in homogeneous phase that leads to only PM1 (dominantly PM0.1) emission, a change from air to oxyfuel results in an increase in PM1 emission as a result of a higher yield of Na, K, S, and P, likely resulting from enhanced sulfation of alkali species under oxyfuel conditions (with a higher O-2 content), but leads to a negligible effect on the release of trace elements (As, Cd, Pb, V, and Zn). On the contrary, for char combustion in heterogeneous phase that contributes to both PM1 and PM1-10 emissions, a change from air to oxyfuel conditions leads to a reduction in PM1 emission but little change in the PM1-10 yield. Such a reduction in PM1 is contributed by reductions in the yields of Na, K, and P, most likely as a result of part of volatilized P to react with CaO to form non-volatile Ca3PO4. For trace elements during char combustion in heterogeneous phase, oxyfuel conditions lead to reductions in As and Cr released as PM1 (most likely as a result of the enhanced formation of Al/Fe/Ca arsenate and iron chromate) but have little effect on the release of Co, Cu, Mn, Ti, and V. The results show that P plays an important role in PM10 emission. For volatiles combustion in homogeneous phase, P is present in PM0.1 (contributes to most of the PM1 formed) in the form of both (Na,K)PO3 and P4O10 that is slightly favored under the oxyfuel conditions as a result of a higher O-2 content. However, for char combustion in heterogeneous phase, P is present in PM0.1 dominantly as (Na,K)PO3, with little P4O10 under both air and oxyfuel conditions. Phosphorus in PM1-10, which is only produced during char combustion, is in the form of Mg-3(PO4)(2) and Ca-3(PO4)(2) under both air and oxyfuel conditions.