The pyrolysis kinetics of cellulose, birch hemicellulose, and birch lignin are studied in a single particle reactor under inert atmosphere. Experiments are carried out with samples of 1-5 mg and mass loss data are measured at constant reactor temperature in the range 225-650degreesC. Arrhenius parameters are determined and are comparable with previous studies. The char yield for cellulose, hemicellulose, and lignin is about 5, 10, and 15%, respectively, at temperatures above 400degreesC. Calculations, where the results for the three independent materials are added together in the proportions of birch wood, give good agreement with birch wood data. This supports the assumption that the three materials act independently during pyrolysis. The results also indicate that above 400degreesC 70% of the char formed during birch wood pyrolysis originates from hemicellulose. visual observations show that lignin melts and boils at high temperatures, whereas hemicellulose expands considerably during the pyrolysis process.