One option to reduce the CO2 emissions is biomass used for energy production. Co-combustion of biomass in existing coal-fired power stations offers a great potential. In pulverized coal combustion facilities, one implementation problem is the tendency of slagging and fouling of biomass. At the IVD, experiments were carried out to obtain information about biomass deposit characteristics in pulverized fuel (pf) co-combustion. This paper presents results from the IVD 0.5 MW pf combustion experimental facility obtained from different tests with coal-biomass mixtures ranging from small biomass shares up to a pure biomass firing. Samples of fuels, the fly ash path, deposition and slagging probes, and slags on furnace walls were taken. The amounts of deposits found on probes inserted into the combustion chamber, their macroscopic characteristics, elemental composition and fusion temperatures were analyzed and compared. Generally, the deposits from the co-combustion experiments softened in ash fusion tests like the main component (coal ash) rather than like a mixture. When co-firing biomass with 25% of thermal input, no slagging was detected on cooled probes with 650 degrees C in the flue gas with 1050 degrees C. Most deposit samples from the probes softened at higher temperatures compared to the laboratory-made ash and as expected for a mixture, but melted as mixtures are expected to. Slagging occurred on uncooled refractory samples, at higher biomass shares, and when the biomass particles were not completely burnt out and sticky when reaching the probes.