The alkali metal release during pyrolysis of biomass is investigated with a surface ionization method. Wheat straw samples (20 mg) are pyrolyzed in a laboratory unit under Nz atmosphere, and two characteristic temperature intervals for alkali metal emission are identified. A small fraction of the alkali metal content is released in a low-temperature region (180-500 degrees C) and is attributed to a connection with the decomposition of the organic structure. The two most pronounced emission processes below 500 degrees C are well described by a first-order rate behavior, and the activation energies are found to be 156 +/- 11 and 178 +/- 8 kJ/mol, The major part of the alkali metal release takes place in the high-temperature region (> 500 degrees C), and activation energies of alkali metal emission from the ash residues are found in the range 168-238 kJ/mol. A high chlorine content is found to enhance the alkali metal emission from the ash, while the alkali metal release in the low-temperature region cannot be correlated with the chlorine content.