Online measurements of individual alkali-containing submicrometer particles in a circulating fluidized bed (CFB) combustion facility are presented. A recently developed particle beam mass spectrometer based on surface ionization technique is demonstrated and its performance during a field campaign in a 12 MW CFB reactor operating under atmospheric conditions is discussed. The boiler was fired with a biomass fuel consisting of a mixture of wood chips and pellets. During part of the campaign K2CO3, HCl, and SO2 were also added to the fuel to investigate their influence on alkali-related processes. Size-selected particles with diameters of 50-300 nm in general had high potassium content, while sodium, rubidium, and cesium were present as minor constituents. Particles formed by recondensation of compounds evaporated during the combustion process could be distinguished from fly ash particles on the basis of their potassium and sodium contents. The number of alkali-containing particles increased when the K, Cl, and S fluxes into the boiler increased, and the trend was confirmed by independent measurements of the total particle concentrations in the flue gas. The concentration of potassium bound to submicrometer particles was found to be in the range 0.8-13 mg/m(3) under the studied operating conditions. When the facility was instead fired with bituminous coal as a fuel, the relative concentration of sodium compared with potassium was higher, and a larger fraction of the alkali compounds were found in fly ash particles. The particle beam mass spectrometer is concluded to accurately provide online information on particle-bound alkali concentrations under the conditions typically prevailing at a commercial-size boiler, and the possibilities for future research and technical development are discussed.