In this work, the sulfur transformations during thermal conversion of two straw samples have been experimentally investigated. Sulfur was found to be associated partly as inorganic sulfate (40-50% of the total S) and partly as organic sulfur (50-40%) in typical Danish wheat straw samples. Batch pyrolysis and combustion experiments were conducted in a lab-scale tubular reactor in order to obtain quantitative information on the sulfur transformations during devolatilization and char burnout. The lab-scale experiments indicated that 35-50% of the total sulfur was released to the gas phase during the devolatilization. The release was predominantly caused by decomposition of organically associated sulfur. During char burnout at low temperature (<500 degreesC), no sulfur was released to the gas phase, but was instead completely retained in the straw ash. As the combustion temperature was increased, sulfur was gradually released to the gas phase; approximately 85% of the total S was released at 950 degreesC. Pyrolysis experiments with SO2 addition indicated that additional sulfur may be fixed in char. Combustion of the sulfur-enriched char samples resulted in significantly higher sulfur concentrations in the residual ashes. On the basis of the experimental results, the transformation and release to the gas phase of biomass sulfur is discussed.