Condensation of potassium species is the origin of the slag layer formation on superheater surfaces during biomass combustion. In this paper, the condensation behaviors of potassium species are studied by a one-dimensional down-fired furnace and a novel condensation probe. Two potassium salts (KCI and K2SO4) and two biomass fuels (wheat straw and corn stalk) are used. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray fluorescence (XRF) analyses were applied to analyze the microstructure and elemental composition of condensation products at different temperatures. The results showed that the condensation temperatures of KCl, K2SO4 and their mixture were 770 +/- 4, 745 +/- 6, and 764 +/- 10 degrees C, respectively. KCl in the initial slag layer was ascribed to the heterogeneous condensation of KCI vapor and the deposition of KCI fine particles that generated by homogeneous nucleation. The pathways were dependent on the wall temperature of the condensation probe. K2SO4 in the initial slag layer was mainly attributed to the deposition of K2SO4-containing fine particles. K2SO4 had much less significant influence on the formation of initial slag layer, compared with KCl. During combustion of biomass fuels, KCl could adhere on the surfaces of fly ash particles, and K2SO4 deposited together with KCl. The condensation behaviors of K2SO4 on fly ash particles were much less remarkable, compared with KCI. The condensation mechanisms of potassium species during biomass combustion were discussed.