In this study, a method for sampling particles and inorganic vapors in hot flue gases was examined in laboratory studies and then applied in field measurements. The method included a hot dilution probe, where the dilution ratio can be controlled. The laboratory results indicated that applied flow rates and dilution ratios can be optimized to produce two distinct particle modes in the particle size distribution: one from the original particles and one from the condensing vapors. The field measurements were performed in a moving grate biomass boiler, and the probe was used for sampling at two positions, at 800 and 1100 degrees C. The size distributions and the size-resolved elemental composition of the samples showed that, at 800 degrees C, alkali sulfates had formed a fine particle mass mode at around 100 nm and that alkali chlorides condensed in the probe to form an additional ultrafine mode in the 10-30 nm range. At 1100 degrees C, a similar bimodal size distribution was obtained as for 800 degrees C; however, the particle diameter of the fine mode was lower and the particle mass concentration was about one-third of the mass at 800 degrees C. The elemental analysis of the ultrafine mode sampled at 1100 degrees C indicated that it was formed mainly from condensing alkali sulfates. In addition, the sample contained detectable amounts of Zn. The suggested interpretation of these results was that, at 1100 degrees C, Zn had oxidized to form ZnO particles and that the formation and condensing of alkali sulfates was in progress, while alkali chlorides were present as vapor.