The effect of co-firing straw and pulverized coal has been investigated in a 2.5 MW(t) pilot-scale burner and a 250 MW(e) utility boiler, In the 2.5 MW(t) trial the straw was chopped and fed separately to the burner, whereas in the full-scale experiment the straw was preprocessed as pellets and ground with the coal in the mills. Straw fractions in the range of 0-20% on a thermal basis were used in the full-scale experiment and 0-100% on a thermal basis in the pilot-scale experiment. Two low-sulfur coals and one high-sulfur coal were used in the pilot scale, whereas a high-sulfur coal was used in full scale. Results from both tests revealed that an increased fraction of straw in the fuel blend resulted in a reduction of NO and SO2 emissions. The lower SO2 emission was partly due to a lower sulfur content of the straw and partly due to retention of sulfur in the ash, probably present as solid alkali sulfates. The reduction of NO emissions was due to lower conversion of the fuel-bound nitrogen. Increasing the straw fraction results in a larger release of volatiles including NO precursors, leading to conditions that can suppress formation of NO from fuel nitrogen.