Applying annual crops as fuel in power plants may give rise to severe deposition in the boilers. in Denmark, straw is applied as fuel in some small combined power and district heating boiler plants. Large differences have been observed in the amount of deposition in the furnace chamber and on superheater tubes when different straw fuel parcels are applied. We have performed 12 experiments at two straw-fired power plants with a fuel input of 23 and 31 MW. in all the experiments the boilers were fed by one type of reasonably uniform straw during an 8 h period. Deposition probes simulating superheater coils were inserted near the superheaters and in the boiler chamber. A long range of parameters was measured, including local temperature, exit flue gas composition, and aerosol particles in the flue gas. Detailed chemical analyses have been performed on the straw, bottom and flue ash, and deposition samples. On the basis of the experimental data the transport of the minerals through the boilers has been followed. For wheat and barley straws the extent of deposition could be correlated with the content of potassium in the straws.