Blends of a South African bituminous "Middleburg" coal, a municipal sewage sludge, and a sawdust have been fired in the slagging reactor to examine the effect of the added fuel on the slagging propensity of the mixtures. Two kinds of deposition probes have been used, uncooled ceramic probes and air-cooled metal probes. to examine the influence of the deposition surface temperature on both the deposit growth and its structure. The initial stages of slagging (140 min of sampling) have been investigated in two temperature ranges: a high-temperature range of 1100-1300 degrees C and a low-temperature range of 550-700 degrees C. Laboratory ash (created in the laboratory furnace), ash sampled on the deposition probes, and ash collected in the cyclone have been analyzed using the X-ray fluorescence technique. Additionally, the electron probe microanalysis (EPMA) of the embedded resin deposit probes have been performed. Using, this technique, the thickness, structure. porosity, and chemical composition in different layers of the deposit have been determined and evaluated as a function of the fuel type and the deposition surface temperature. Distinct differences in structures of the deposits collected using the uncooled ceramic probes and air-cooled steal probes have been observed. Glassy, easily molten deposits collected on uncooled ceramic deposition probes are characteristic for co-firing of municipal sewage sludge with coal. Porous, sintered (not molten), but easily removable deposits of the same fuel blend have been collected on the air-cooled metal deposition probes. The addition of sawdust does not negatively influence the deposition behavior. Loose, easy removable deposits have been sampled on air-cooled metal deposition probes during co-firing of coal-sawdust blends. The mass of the deposit sampled at lower deposition surface temperatures (550-700 degrees C) was always larger than the mass sampled at higher Surface temperatures (1100-1300 degrees C).