The emission of potassium- and sodium-containing compounds during rapid birchwood pyrolysis was studied. Birchwood particles (2−130 mg) were inserted into a preheated furnace at constant temperature (350−850 °C) and the alkali emission was measured. Particle mass, furnace temperature, and moisture were varied. At temperatures ≤ 500 °C, the alkali emission from birchwood particles took place solely during the pyrolysis phase. At temperatures ≥ 600 °C, alkali evaporation from the ash increased. The total alkali release increased with temperature in the range studied and the release during the pyrolysis was larger or equal to the release from the ash phase. Small particles (2−10 mg) emitted more alkali per unit mass than large ones (60−130 mg) and this tendency increased with temperature. At 800 °C the emission per unit mass from small particles was 10 times the one for large particles. Wet particles went through a drying phase, which delays the heating, and thereby the alkali emission. The present findings are of importance for actions aimed at minimizing alkali related problems during large-scale biomass conversion.