In the present study the influence of different fuel blends and additives on the release of gaseous alkali metals during rapid pyrolysis of biomass has been investigated. A single particle reactor has been used together with a molecular beam mass spectrometer (MBMS) to study the release of gaseous alkali-containing emissions during pyrolysis. A hot platinum filament was used as the ionization source and alkali-containing compounds that hit the hot filament dissociate and alkali ions leave the Pt -filament and are detected in the MBMS. The fuels used were wood (spruce) and wood waste. The additives included peat, different sewage sludge samples, sludge from the pulp and paper industry and fly ash from co-combusted wood and sewage sludge. Experimental results show time resolved mass loss curves of biomass particles during rapid pyrolysis combined with released amounts of sodium (Na) and potassium (K). The results indicate that the new combined instrument successfully detects gaseous alkali metals, and a difference in the release of Na and K during pyrolysis due to the use of the additives is observed. Additives consisting of sewage sludge ash reduced the release of alkali, probably due to the presence of mullite in the ash together with the sorption effect of fine particles.