A conventional pc-fired boiler at the Danish energy company I/S Midtkraft has been converted to coal-straw co-combustion, and a 2 year demonstration program was initiated in January 1996, addressing several aspects of coal-straw co-combustion. Deposition trials were performed as part of the demonstration program. A maximum straw share of approximately 20% (energy base) was used in the experiments. For the deposit samples collected, a visual analysis procedure was developed and each sample evaluated according to this. In addition, a number of samples were analyzed by scanning electron microscopy (SEM) combined with energy dispersive X-ray analyses (SEM-EDX) and bulk chemical analyses. In the visual analysis, a significant increase in the amount and tenacity of the upstream deposits was observed as a function of increased straw share, exposure time, and boiler load. The chemical analyses of the deposits show increased amounts of K and S during co-combustion, and the Fe-dominated upstream deposits formed during coal combustion are shifted toward more Ca- and Si-rich deposits during coal-straw cocombustion. However, the major part of K is observed to form K-Al silicates, which do not form problematic deposits. Go-firing straw also caused a change in the structure of the upstream deposits. During coal combustion an ordered, "finger" structure of the larger particles with small particles between was observed, whereas during co-combustion a more random deposition of the larger particles among the small ones was observed. No chlorine species was observed in the deposits collected, and selective chlorine corrosion is therefore not expected to constitute a problem in co-combustion of coal and straw up to 20% straw share, for the coal types utilized in the tests. However, deposition problems could arise when burning other coals, particularly coals with a high S or alkali metal content or a low content of ash. The behavior of K, Ca, S, and Cl was evaluated by use of thermodynamic calculations. The thermodynamically stable species agree with the observed behavior in the experiments, i.e. formation of stable K-Al silicate species as well as KxO4 is predicted. The calculations also emphasize that the mixing between the coal and straw species is essential for the deposition behavior, primarily by affecting the split between K-Al silicates and K2SO4.