The co-combustion of pulverized-biomass in a natural gas-fired large-scale laboratory furnace is investigated experimentally. The biomass fuels used included pine sawdust, pine shells, pine branches, and olive stones. For comparison purposes, the co-combustion tests have also included the study of natural gas/pulverized-coal flames. For each solid-fuel, flue-gas data were obtained for pollutant emissions and particle burnout as a function of the solid-fuel cofiring ratio. Ratios up to 20% (energy basis) were used in the experiments. Subsequently, in order to improve the understanding of the processes that occur during the co-combustion of natural gas and solid fuels, the study focused on detailed in-flame measurements of major gas species, including NOx, and gas temperatures for three representative furnace operating conditions. The results show that (1) cofiring of biomass fuels, compared with pure natural gas. ring, has no significant effect on NOx emissions provided that the fuel-bound nitrogen is low because of the positive impact of the reduced in-flame temperatures of the cofiring in the thermal NO formation that serves to attenuate, to some extent, the fuel NO formation associated with biomass fuels; (2) cofiring of biomass fuels, compared with pure natural gas. ring, may have an important impact on CO and hydrocarbon emissions if particle sizes are too large because this extends the volatilization process downstream into regions with relatively low temperatures; and (3) cofiring of biomass fuels, compared with cofiring of coal, allows for much larger particle sizes without a deleterious effect on particle burnout performance due to its higher content of volatile matter.