Particulate and gaseous emissions were studied in a 7 MW moving-grate boiler fired with moist sawmill residues together with varying admixtures of elementary sulfur. The particle number concentration decreased by approximately 25% and the sub-micrometer particle mass concentration increased by approximately 100% as 0.17% (by mass, dry substance) of elementary sulfur was added to the fuel. Sulfur addition also resulted in a shift towards larger particle mean diameters. Elementary analysis indicated that the amount of particle-bound sulfur increased significantly with sulfur addition, probably by favoring the formation of alkali sulfates. CO emissions were reduced by approximately 50% in the measurement period when the sulfur admixture with the fuel was 0.17% or higher. The SO2 concentration measured downstream from the flue gas condenser was below the detection limit when no sulfur was admixed and was approximately 30 ppm when 0.12% elementary sulfur was admixed. Sulfur admixture had no significant effect on the NOx concentration.